Support for image URI's

[GoogleCodeExporter]

What version of CssMin are you using (source and/or build)?
v3.0.1

What was the input stylesheet and/or configuration options?
background-image: 
url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAYAAAAGCAYAAADgzO9IAAAACXBIW
XMAAAsTAAALEwEAmpwYAAAAB3RJTUUH3QMbCgIjPwqZOQAAAClJREFUCNdj/Pz5838GLICFgYGB4dev3
yiCbGysDEwMOADpEiwwM9EBAKGJBvnRqy0+AAAAAElFTkSuQmCC');

What is the expected result?
background-image: 
url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAYAAAAGCAYAAADgzO9IAAAACXBIW
XMAAAsTAAALEwEAmpwYAAAAB3RJTUUH3QMbCgIjPwqZOQAAAClJREFUCNdj/Pz5838GLICFgYGB4dev3
yiCbGysDEwMOADpEiwwM9EBAKGJBvnRqy0+AAAAAElFTkSuQmCC');

And what is the actual result and/or error message?
background-image: 
url('../css/data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAYAAAAGCAYAAADgzO9IAA
AACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH3QMbCgIjPwqZOQAAAClJREFUCNdj/Pz5838GLICFgY
GB4dev3yiCbGysDEwMOADpEiwwM9EBAKGJBvnRqy0+AAAAAElFTkSuQmCC');

Please provide any additional information below.
As URI usage is getting more often used, so this option is mandatory for me, 
and I guess, many others.

Original issue reported on code.google.com by Nils.Lindentals on 10 May 2013 at 7:44