Compass Sprite imports break KSS parsing

* Compass Sprite imports can be of the form `@import "folder/*.png"`.
* This matches the regex for multi-line comments and the rest of the
  file would then be ignored.
* New behaviour is to `continue` if we aren't already in a multi-line
  comment if the line starts with @import.
* Ran mocha test suite and still passed.
* Manually verified as well.