Run pull request filters concurrently

Before: pulls --mine  2.26s user 1.33s system 18% cpu 19.892 total
After: pulls --mine  2.30s user 1.06s system 64% cpu 5.239 total

Added a sorter to make sure the output is actually sorted (like it was
before). Maybe this should move elsewhere?
Merge pull request #156 from cpuguy83/run_pull_filters_concurrently

Run pull request filters concurrently