Add paths option in local-plugins config file.
Closes #379 (closed).
The most complex part of the implementation turned out to be allowing for paths to be relative to the config file they are in, but I felt this was critical for usability.
This implementation isn't perfect; if there are multiple local config files, we actually allow the path to be relative to any of them, regardless of which one the paths option was provided in. But this was the best I could do without doing even more invasive surgery on the config system to preserve exactly which config file each option was found in.
Let me know if you have comments! It'd be sweet if this could sneak into 3.5.0...