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

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](https://docs.python.org/3.8/library/configparser.html#interpolation-of-values). 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.