Merge branch 'feature.custom-highlighting' into 'master'

add tests for .gitattributes custom highlighting

This depends on being in master because rugged's `.gitattributes` implementation requires the `.gitattributes` file to be on HEAD. But @stanhu mentioned that there are tests that will break if we add stuff to master in this repo.

See merge request !9

.gitattributes

+/custom-highlighting/*.gitlab-custom gitlab-language=ruby

custom-highlighting/test.gitlab-custom

+class ShouldBeHighlightedAsRuby
+  def because_of_gitattributes
+    cool!
+  end
+end