Add option to filter graphed OnionPerf results by relay fingerprint

We want to provide the option to filter OnionPerf measurements by relay fingerprint to see how overall performance would change when removing relays with certain properties. The idea is that the user provides a set of fingerprints, either as command-line arguments or contained in a flat file, and we exclude all measurements with those relays in the path. Ideally, we would make it possible for different sets of excluded relays to be plotted as different CDFs in the same graph.