Upgrade weather-dl config language to allow updates from continuous integration services.

[alxmrs]

Right now, weather-dl and it's configs are oriented towards manual invocation. It would be quite useful to be able to run this tool in a CI, where CLI arguments would stay the same and the config would change.

Implementation notes

A simple way to implement this could be to just upgrade our ConfigParser class to one that supports interpolation. It looks like the majority of the capabilities we want already exist with the default parser, however, it would be best to make sure this doesn't interfere with other language features.