added data-uri support

As it states in http://lesscss.org/#reference

Inlines a resource and falls back to url() if the ieCompat option is on and the resource is too large, or if you use the function in the browser. If the mime is not given then node uses the mime package to determine the correct mime type.

Parameters:

mimetype: A mime type string. Optional.
url: The URL of the file to inline.