Added font support and made configuration more robust.

Hi Ben,

This pull request contains somewhat specific changes for a project I am involved in. I suspect you will want to deny this request, but feel free to integrate any changes you find worthwhile.

I added "font" support in addition to "theme" support to create X*Y CSS permutations.  The following example would product 4 output files:

themes/
--light.less
--dark.less

fonts/
--fontA.less
--fontB.less

output:

theme-light-fontA.less
theme-light-fontB.less
theme-dark-fontA.less
theme-dark-fontB.less

If fonts are not defined, only themes are compiled. In addition, I added globbing support for both themes and fonts and cleaned up the output to be more similar to other grunt plugins.

Let me know if you have any questions or would like me to make any changes. 

- Jon, Perfect Sense Digital