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

test/handler/test_ruby_handler.vader

@@ -24,7 +24,7 @@ Execute(The ruby handler should parse lines correctly and add the column if it c
   \     'text': 'syntax error, unexpected end-of-input, expecting keyword_end'
   \   }
   \ ],
-  \ ale_linters#ruby#ruby#Handle(255, [
+  \ ale#handlers#ruby#HandleSyntaxErrors(255, [
   \ "test.rb:6: syntax error, unexpected ';'",
   \ "        t = ;",
   \ "             ^",