[eruby] Add erubi linter

Erubi is yet another parser for eRuby. This is the default parser in
Rails as of version 5.1. It supports some additional syntax with similar
behavior to Rails' extensions to the language, though incompatible.
Rails currently still recommends their own syntax, so GetCommand still
has to do the translation introduced in
https://github.com/w0rp/ale/pull/1114 .

Erubi does not supply an executable—It is intended to be invoked only
from within a Ruby program. In this case, a one-liner on the command
line.

test/command_callback/test_erubi_command_callback.vader

@@ -0,0 +1,31 @@
+Before:
+  runtime ale_linters/eruby/erubi.vim
+  call ale#test#SetDirectory('/testplugin/test/command_callback')
+
+After:
+  call ale#linter#Reset()
+  call ale#test#RestoreDirectory()
+
+Execute(Executable should not contain any filter code by default):
+  call ale#test#SetFilename('../ruby_fixtures/not_a_rails_app/file.rb')
+
+  AssertEqual
+  \ 'ruby -r erubi/capture_end -e ' . ale#Escape('puts Erubi::CaptureEndEngine.new($stdin.read).src') . '< %t | ruby -c',
+  \ ale_linters#eruby#erubi#GetCommand(bufnr(''), [])
+
+Execute(Executable should filter invalid eRuby when inside a Rails project):
+  call ale#test#SetFilename('../ruby_fixtures/valid_rails_app/app/views/my_great_view.html.erb')
+
+  AssertEqual
+  \ 'ruby -r erubi/capture_end -e ' . ale#Escape('puts Erubi::CaptureEndEngine.new($stdin.read.gsub(%{<%=},%{<%}), nil, %{-}).src') . '< %t | ruby -c',
+  \ ale_linters#eruby#erubi#GetCommand(bufnr(''), [])
+
+Execute(Command should be blank if the first command in the chain return output):
+  let output_lines = [
+  \    "/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- erubi/capture_end (LoadError)",
+  \    "        from /usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'",
+  \]
+
+  AssertEqual
+  \ '',
+  \ ale_linters#eruby#erubi#GetCommand(bufnr(''), output_lines)