Add glitch follow-up plugin

This merge request allows follow-up of lock loss times using glitch-based triggers

Additions:

• plugins/glitch.py: module for glitch follow-up. It has two functions in here that I borrowed from iDQ and until there's a standard way to read in these triggers. I assume that path will be through gwpy since it is already able to ingest other glitch-based triggers than from the GstLAL-based one.
• plugins/__init__.py: glitch plugin registration. Should this be an optional follow-up tool, and how would this be incorporated if so?
• config.py: Register GSTLAL_TRIGGER_DIR as a config setting. This should most likely be optional as well, I don't like having this be required but I'll wait for your input before I change anything.