Adds standardrb linter (#2133)

See: https://github.com/testdouble/standard

StandardRB is to RuboCop what StandardJS is to ESLint. This commit 
naively copies the RuboCop linter and fixer to point at the standardrb
executable. Any other adjustments are very minor (the only I can think 
of is that standardrb takes a `--fix` option instead of 
`--auto-correct`).

This raises a confusing point to me as both developer and a user: since
ale enables all linters by default, won't this run both RuboCop and 
StandardRB (the results of which will almost always be in conflict with
one another)? How does ale already solve for this for the similar case
of StandardJS and ESLint?

test/command_callback/test_standardrb_command_callback.vader

@@ -0,0 +1,29 @@
+Before:
+  call ale#assert#SetUpLinterTest('ruby', 'standardrb')
+  call ale#test#SetFilename('dummy.rb')
+
+  let g:ale_ruby_standardrb_executable = 'standardrb'
+  let g:ale_ruby_standardrb_options = ''
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(Executable should default to standardrb):
+  AssertLinter 'standardrb', ale#Escape('standardrb')
+  \   . ' --format json --force-exclusion  --stdin '
+  \   . ale#Escape(ale#path#Simplify(g:dir . '/dummy.rb'))
+
+Execute(Should be able to set a custom executable):
+  let g:ale_ruby_standardrb_executable = 'bin/standardrb'
+
+  AssertLinter 'bin/standardrb' , ale#Escape('bin/standardrb')
+  \   . ' --format json --force-exclusion  --stdin '
+  \   . ale#Escape(ale#path#Simplify(g:dir . '/dummy.rb'))
+
+Execute(Setting bundle appends 'exec standardrb'):
+  let g:ale_ruby_standardrb_executable = 'path to/bundle'
+
+  AssertLinter 'path to/bundle', ale#Escape('path to/bundle')
+  \   . ' exec standardrb'
+  \   . ' --format json --force-exclusion  --stdin '
+  \   . ale#Escape(ale#path#Simplify(g:dir . '/dummy.rb'))

test/fixers/test_standardrb_fixer_callback.vader

@@ -0,0 +1,54 @@
+Before:
+  Save g:ale_ruby_standardrb_executable
+  Save g:ale_ruby_standardrb_options
+
+  " Use an invalid global executable, so we don't match it.
+  let g:ale_ruby_standardrb_executable = 'xxxinvalid'
+  let g:ale_ruby_standardrb_options = ''
+
+  call ale#test#SetDirectory('/testplugin/test/fixers')
+  silent cd ..
+  silent cd command_callback
+  let g:dir = getcwd()
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(The standardrb callback should return the correct default values):
+  call ale#test#SetFilename('ruby_paths/dummy.rb')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Escape(g:ale_ruby_standardrb_executable)
+  \     . ' --fix --force-exclusion %t',
+  \ },
+  \ ale#fixers#standardrb#Fix(bufnr(''))
+
+Execute(The standardrb callback should include configuration files):
+  call ale#test#SetFilename('ruby_paths/with_config/dummy.rb')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Escape(g:ale_ruby_standardrb_executable)
+  \     . ' --config ' . ale#Escape(ale#path#Simplify(g:dir . '/ruby_paths/with_config/.standard.yml'))
+  \     . ' --fix --force-exclusion %t',
+  \ },
+  \ ale#fixers#standardrb#Fix(bufnr(''))
+
+Execute(The standardrb callback should include custom rubocop options):
+  let g:ale_ruby_standardrb_options = '--except Lint/Debugger'
+  call ale#test#SetFilename('ruby_paths/with_config/dummy.rb')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Escape(g:ale_ruby_standardrb_executable)
+  \     . ' --config ' . ale#Escape(ale#path#Simplify(g:dir . '/ruby_paths/with_config/.standard.yml'))
+  \     . ' --except Lint/Debugger'
+  \     . ' --fix --force-exclusion %t',
+  \ },
+  \ ale#fixers#standardrb#Fix(bufnr(''))

test/handler/test_rubocop_handler.vader

@@ -41,21 +41,21 @@ Execute(The rubocop handler should parse lines correctly):
   \     'type': 'E',
   \   },
   \ ],
-  \ ale_linters#ruby#rubocop#Handle(347, [
+  \ ale#ruby#HandleRubocopOutput(347, [
   \     '{"metadata":{"rubocop_version":"0.47.1","ruby_engine":"ruby","ruby_version":"2.1.5","ruby_patchlevel":"273","ruby_platform":"x86_64-linux-gnu"},"files":[{"path":"my_great_file.rb","offenses":[{"severity":"convention","message":"Prefer single-quoted strings...","cop_name":"Style/SomeCop","corrected":false,"location":{"line":83,"column":29,"length":7}},{"severity":"fatal","message":"Some error","cop_name":"Style/SomeOtherCop","corrected":false,"location":{"line":12,"column":2,"length":1}},{"severity":"warning","message":"Regular warning","cop_name":"Style/WarningCop","corrected":false,"location":{"line":10,"column":5,"length":8}},{"severity":"error","message":"Another error","cop_name":"Style/SpaceBeforeBlockBraces","corrected":false,"location":{"line":11,"column":1,"length":1}}]}],"summary":{"offense_count":4,"target_file_count":1,"inspected_file_count":1}}'
   \ ])
 
 Execute(The rubocop handler should handle when files are checked and no offenses are found):
   AssertEqual
   \ [],
-  \ ale_linters#ruby#rubocop#Handle(347, [
+  \ ale#ruby#HandleRubocopOutput(347, [
   \     '{"metadata":{"rubocop_version":"0.47.1","ruby_engine":"ruby","ruby_version":"2.1.5","ruby_patchlevel":"273","ruby_platform":"x86_64-linux-gnu"},"files":[{"path":"my_great_file.rb","offenses":[]}],"summary":{"offense_count":0,"target_file_count":1,"inspected_file_count":1}}'
   \ ])
 
 Execute(The rubocop handler should handle when no files are checked):
   AssertEqual
   \ [],
-  \ ale_linters#ruby#rubocop#Handle(347, [
+  \ ale#ruby#HandleRubocopOutput(347, [
   \     '{"metadata":{"rubocop_version":"0.47.1","ruby_engine":"ruby","ruby_version":"2.1.5","ruby_patchlevel":"273","ruby_platform":"x86_64-linux-gnu"},"files":[],"summary":{"offense_count":0,"target_file_count":0,"inspected_file_count":0}}'
   \ ])
 
@@ -66,11 +66,11 @@ Execute(The rubocop handler should handle output without any errors):
 
   AssertEqual
   \ [],
-  \ ale_linters#ruby#rubocop#Handle(347, g:lines)
+  \ ale#ruby#HandleRubocopOutput(347, g:lines)
   \
   AssertEqual
   \ [],
-  \ ale_linters#ruby#rubocop#Handle(347, ['{}'])
+  \ ale#ruby#HandleRubocopOutput(347, ['{}'])
   AssertEqual
   \ [],
-  \ ale_linters#ruby#rubocop#Handle(347, [])
+  \ ale#ruby#HandleRubocopOutput(347, [])