Print line if we have not seen any pids created yet

This specifically handles the case where you get a log snippet that may
not have any pid creation lines in it and pidcat ends up filtering
everything out. In this case we just print the line if we haven't seen
any pids yet.

An alternative would be allowing filter by pid, however I find grep
works fine for that.
Merge pull request #67 from brennantaylor/print_if_no_pids

Print line if we have not seen any pids created yet