Restrict import processor to .styl files

Currently, ruby-stylus doesn't interact well with files that have the same syntax for `@import`s, but aren't stylus files, such as SASS files. In some cases, this can cause really nasty side effects - we sometimes saw endless loops processing our legacy SASS templates that led to the Stylus code.

This change should restrict the @import directive processor to .styl files, and really helps us smoothly transition our stylesheets over. I hope you'll consider it for inclusion! (-: