[request] Support behavior change via config file

[hinto-janai]

This is a request for P2Pool to support behavior changes via a config file rather than command line arguments, like XMRig.

PR changes:

1. Add command line arguments in JSON-form to config.json
2. Create a regular non-JSON config file: p2pool.conf
3. Add a Configuration section to README

The config files have the current v2.2.1 options and the values are set to the current defaults. I can remove and squash if JSON or conf is preferred. The README has a placeholder for the config name too.