Add linter for Inko

This adds a linter for Inko (https://inko-lang.org/). The linter makes
use of Inko's own compiler, and a newly introduced --check flag to only
check for errors; instead of also compiling source code.

test/command_callback/test_inko_inko_callbacks.vader

@@ -0,0 +1,20 @@
+Before:
+  call ale#assert#SetUpLinterTest('inko', 'inko')
+  call ale#test#SetFilename('inko_paths/test.inko')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  AssertLinter 'inko', ale#Escape('inko') . ' build --check --format=json %s'
+
+Execute(The inko callback should include tests/ for test paths):
+  call ale#engine#Cleanup(bufnr(''))
+  noautocmd e! inko_paths/tests/test/test_foo.inko
+  call ale#engine#InitBufferInfo(bufnr(''))
+
+  AssertLinter 'inko',
+  \ ale#Escape('inko')
+  \   . ' build --check --format=json --include '
+  \   . ale#Escape(ale#path#Simplify(g:dir . '/inko_paths/tests/'))
+  \   . ' %s'

test/handler/test_inko_handler.vader

@@ -0,0 +1,54 @@
+Before:
+  runtime ale_linters/inko/inko.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute(The inko handler should parse errors correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'filename': ale#path#Simplify('/tmp/foo.inko'),
+  \     'lnum': 4,
+  \     'col': 5,
+  \     'text': 'this is an error',
+  \     'type': 'E',
+  \   }
+  \ ],
+  \ ale#handlers#inko#Handle(bufnr(''), [
+  \ '[',
+  \ '  {',
+  \ '    "file": "/tmp/foo.inko",',
+  \ '    "line": 4,',
+  \ '    "column": 5,',
+  \ '    "message": "this is an error",',
+  \ '    "level": "error"',
+  \ '  }',
+  \ ']'
+  \ ])
+
+Execute(The inko handler should parse warnings correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'filename': ale#path#Simplify('/tmp/foo.inko'),
+  \     'lnum': 4,
+  \     'col': 5,
+  \     'text': 'this is a warning',
+  \     'type': 'W',
+  \   }
+  \ ],
+  \ ale#handlers#inko#Handle(bufnr(''), [
+  \ '[',
+  \ '  {',
+  \ '    "file": "/tmp/foo.inko",',
+  \ '    "line": 4,',
+  \ '    "column": 5,',
+  \ '    "message": "this is a warning",',
+  \ '    "level": "warning"',
+  \ '  }',
+  \ ']'
+  \ ])
+
+Execute(The inko handler should handle empty output):
+  AssertEqual [], ale#handlers#inko#Handle(bufnr(''), [])