Allow passing of additional context to template.

This branch adds the ability to pass additional context to a template.

The reason for this is to be able to pass the spritePath and possibly other vars to a template in grunt-cssspritesmith so that the CSS output can be optimised to allow setting the background image in a separate selector and then overriding just the background position on a case by case basis.

In addition this would mean a template author can have finer-grained control over the output.