Peakrdl

**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)

Initial release!