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

ale_linters/eruby/erubylint.vim

@@ -0,0 +1,11 @@
+" Author: Matthias Guenther - https://wikimatze.de
+" Description: erb-lint for eruby/erb files
+
+call ale#linter#Define('eruby', {
+\   'name': 'erubylint',
+\   'executable': 'erb',
+\    'output_stream': 'stderr',
+\   'command': 'erb -P -x %t | ruby -c',
+\   'callback': 'ale#handlers#ruby#HandleSyntaxErrors',
+\})
+