3rd Party Bundle Support

Today (2021-09-08) Circup can download and install libraries from three bundles:

- Adafruit CircuitPython Bundle
- Community Bundle
- CircuitPython.org Bundle

This issue\discussion is about how to support 3rd party bundles which follow the CP library bundling format. This conversation originally started in a PR to move the references to the bundles from within the code to an JSON configuration file. (https://github.com/adafruit/circup/pull/126)

Proposals at that time included:

## User Supplied Config.json

In addition to the config.json released with circup, look for a config file in [AppDirs](https://pypi.org/project/appdirs/) so that you can look for the config json file first in the user's configuration directory and then to the default list shipped in the package. If a config.json existed in AppDirs, those bundles would be merged with the bundles released with Circup.

## Circup Bundle Management Commands

Add Bundle Management Commands to Circup for _Add_, _Remove_, _Reorder_, display _Info_ for bundles. This would update the configuration JSON.

> I think that summarizes things.. I'll add my own comments\thoughts seperately

Review PR #126 and the shipped config.json approach first, along with the AppDirs proposal. Resolve whether user-supplied configuration, bundle management commands, or both are required, including how bundles are added, removed, reordered, and inspected. Done means Circup can support third-party bundles that follow the CircuitPython library bundling format.

Written by the indexing model from the issue text.