pmc: add filter command

pmc filter allows the user to select event types, threads, and processes from
a sample.

% pmcstat -S unhalted_core_cycles -S llc-misses -S -S resource_stalls.any -O pmc.log
% pmc filter -e llc-misses pmc.log pmc-llc-misses.log
% pmc filter -e unhalted_core_cycles -t 100339 pmc.log pmc-core-cycles.log
etc...
% pmcstat -R pmc-core-cycles.log -G pmc-core-cycles.stacks