Add incremental filtering.

This is the work of Seth Heeren from this patch: http://sourceforge.net/p/htop/patches/17/

It adds the option to filter the process list rather than just search. The rest of the discussion on that patch makes some small changes to this code so I'm really only opening this PR in the hope that someone with more C experience could help test and implement this feature, as I would love to have the ability to filter.