Allow overriding rubocop executable.

test/command_callback/test_rubocop_command_callback.vader

@@ -0,0 +1,29 @@
+Before:
+  runtime ale_linters/ruby/rubocop.vim
+
+Execute(Executable should default to rubocop):
+  AssertEqual
+  \ 'rubocop --format emacs --force-exclusion  --stdin ''dummy.py''',
+  \ ale_linters#ruby#rubocop#GetCommand(bufnr(''))
+
+Execute(Should be able to set a custom executable):
+  let g:ale_ruby_rubocop_executable = 'bin/rubocop'
+  AssertEqual
+  \ 'bin/rubocop --format emacs --force-exclusion  --stdin ''dummy.py''',
+  \ ale_linters#ruby#rubocop#GetCommand(bufnr(''))
+
+Execute(Custom executables should not be escaped):
+  let g:ale_ruby_rubocop_executable = 'bundle exec rubocop'
+  AssertEqual
+  \ 'bundle exec rubocop --format emacs --force-exclusion  --stdin ''dummy.py''',
+  \ ale_linters#ruby#rubocop#GetCommand(bufnr(''))
+
+Execute(Executable callback should return the first token of the executable):
+  let g:ale_ruby_rubocop_executable = 'bundle exec rubocop'
+  AssertEqual
+  \ 'bundle',
+  \ ale_linters#ruby#rubocop#GetExecutable(bufnr(''))
+  let g:ale_ruby_rubocop_executable = 'bin/rubocop'
+  AssertEqual
+  \ 'bin/rubocop',
+  \ ale_linters#ruby#rubocop#GetExecutable(bufnr(''))