Feature: Support for filter tag

I made an attempt to support this in the cleanest way I could with the current parsing/function setup. It isn't especially gross, but it still feels way less elegant than the other stuff I've seen. I'd love for any ideas on a cleaner way to implement this.

I saw in one of the other issues, there is potentially an update to functions in the works to helps support things like parent() & block(). If that is true, it may not be worth merging this in since I feel like it is going about things in a slightly convoluted way to make it work.

http://twig.sensiolabs.org/doc/tags/filter.html