Add erb linter (#497)

* Add eruby linter

* Update README with erb linter

* Fix example and contributions

* Remove trailing newline

* Fix for Vimscript style guide

* Eruby-linter: codereview with @w0rp
- read from stderro output_stream

* Eruby-linter: codereview => add handler for ruby

* Eruby-linter: codereview
- eruby and ruby lint use the same ruby-handler (removes
  duplicated handling logic)

* Eruby-linter: try to fix tests

autoload/ale/handlers/ruby.vim

@@ -0,0 +1,37 @@
+" Author: Matthias Guenther https://wikimatze.de
+"
+" Description: This file implements handlers specific to Ruby.
+
+function! s:HandleSyntaxError(buffer, lines) abort
+    " Matches patterns line the following:
+    "
+    " test.rb:3: warning: parentheses after method name is interpreted as an argument list, not a decomposed argument
+    " test.rb:8: syntax error, unexpected keyword_end, expecting end-of-input
+    let l:pattern = '\v^.+:(\d+): (warning: )?(.+)$'
+    let l:column = '\v^(\s+)\^$'
+    let l:output = []
+
+    for l:line in a:lines
+        let l:match = matchlist(l:line, l:pattern)
+        if len(l:match) == 0
+            let l:match = matchlist(l:line, l:column)
+            if len(l:match) != 0
+                let l:output[len(l:output) - 1]['col'] = len(l:match[1])
+            endif
+        else
+            call add(l:output, {
+            \   'lnum': l:match[1] + 0,
+            \   'col': 0,
+            \   'text': l:match[2] . l:match[3],
+            \   'type': empty(l:match[2]) ? 'E' : 'W',
+            \})
+        endif
+    endfor
+
+    return l:output
+endfunction
+
+function! ale#handlers#ruby#HandleSyntaxErrors(buffer, lines) abort
+    return s:HandleSyntaxError(a:buffer, a:lines)
+endfunction
+