Add support for erblint

[roelandmoors]

Add support for erblint as asked in issue #3911

[hsanson]

Add a test for the handler method. See examples inside the test/handler folder.

[hsanson]

Thanks, looks good.

[evenreven]

(Dumb) question: I tried it now and it works well (thank you both!), but is it meant to autofix as well the way the Rubocop linter/fixer does? Autofixing is supported by erblint, and the VS Code extension does it as well, but I get the There is no fixer named 'erblint' error when I try ALEFix. I have this in my .vimrc:

let g:ale_fixers = {
      \ '*': ['remove_trailing_lines', 'trim_whitespace'],
      \'scss': ['stylelint'],
      \'css': ['stylelint'],
      \'ruby': ['rubocop'],
      \'eruby': ['erblint'],
      \'javascript': ['prettier', 'eslint'],
      \'json': ['prettier'],
      \'slim': ['slim-lint'],
      \'yaml': ['prettier'],
      \'html': ['prettier'],
      \}

If it's out of scope for this PR, that's perfectly understandable.

[hsanson]

@evenreven unfortunatelly this PR only adds erblint as a linter. Supporting it as fixer should not be difficult. Check the ale/autoload/ale/fixers/rubocop.vim file for a simple example.

[roelandmoors]

yes, I only created the linter. Realized too late it could be used as fixer. Maybe I'll still add this if I find some time.