add slim support

[Slim](http://slim-lang.com) is a popular template language. I think wiredep should support it out-of-the-box.

From [its doc](http://www.rubydoc.info/gems/slim/frames#Code_comment__), there are two kinds of comments:

HTML comment:  `/! comment`
And code comment: `/ comment`

It supports both with this PR.  Test cases are also added.