Various bug fixes and new features

Output is now lexicographically sorted in both versions of porthos. New
features in porthos.py have now been tested and debugged. Creating large
graphs can take a long time (an hour for millions of edges), so the name-based
filtering features have also been reimplemented in a stand-alone program,
filter_graph.py.

Signed-off-by: Ted Gibbons <ted.codes@gmail.com>
Merge branch 'master' of https://github.com/trgibbons/porthos

I was ineffectively multi-tasking and used commit --amend on the wrong blob