Allow use of _ convention from Sass.

Sass has the convention of prepending @import files with _. Rails doesn't
enforce this convention but it does allow for it. This helps make the pipeline
more compatible with Sass libraries that name it's files based on this Sass
convention.
Merge branch 'working_at_import' into try_underscored_file
Merge branch 'master' into try_underscored_file
Merge pull request #27 from eric1234/try_underscored_file

Try underscored file