track dependent files and force recompilation if they change

this only is relevant for less/sass files where @import is resolved at
compilation time
update docs a bit
use knowledge of parent file type for additional logic

- only do the mixin stuff (_ filenames) for sass and less
- only check dependencies for @import'ed .less files
- don't look at the file extension, but at the specified mime type.
  This is in accordance to the rest of sacy