custom css template support

I discovered I could really use a custom template option, solely for customizing the @font-face declaration (specifying order of font types and customizing the path to trigger converting one of the fonts to a data-uri via our asset packager).

What I implemented here is limited but should be useful.

* custom css template specified via '-t your_custom.css'
* 'your_custom.css' is located in same dir as vector files (it's a modified version of the default template)
 
I'm not assuming any changes to the main or ie7 css, which I think work well. It's only the font-face where I can see a need for customization. 

Alternately, there's probably a way to specify font ordering and base64-encoding data-uri's via cli options, but I thought this was the simplest approach to start.