PR to close Issues/50

Hello,

I wanted to use some jade-filters in my project, but no one grunt-plugins had no custom-filter support.
I fixed #50 for myself and want to help others. By the way I know that [eval is evil][1], but don't know the better way to solve this issue, that's why I'm open for suggestions and healthy criticism.

[1]: http://www.xkr.us/js/eval