Peakrdl

**Updates:**
* Add workaround to bug in py3.8/py3.9's distribution metadata discovery that impacts ability to load plugins. 16

**Updates:**
* Rework plugin framework to rely on plugins extending from base class.
* Add support for TOML-based PeakRDL configuration file.

**Updates:**
* Add `preprocess` command (12)
* Update plugin dependencies to include new SystemRDL exporter

**Updates:**
* Fix `-f FILE` argument loading feature. 11

**Updates:**
* Fix argparse compatibility issue with Python 3.6 (6 - Thanks proukema-fidus!)
* Add hooks for subcommand plugins to be able to register their own UDPs

**Updates:**
- Fix entry point discovery for older versions of python (5)