Added filter groups to speed up "is:" and "ext:"

Adds an Ack::Filter::Collection class that can contain filters and
internally sort them into groups. The groups can then be optimized
for faster filtering.

Currently "is" and "ext" filters are grouped and replaced by a fast
hash lookup. This leads to improved performance when many such filters
are active, like when using the --known command line option.
Merge pull request #375 from sth/grouped-filters

Added filter groups to speed up "is:" and "ext:"