Create a plugin system for custom checks

Pint has an excellent set of checks with good defaults. However some users may want to add their own plugins with custom business logic or weird, opinionated plugins that don't fit well in this repository. I propose adding a plugin system similar to [slok/sloth's new plugin system](https://sloth.dev/usage/slo-plugins/#developing-plugins) which uses [Yaegi](https://github.com/traefik/yaegi) so plugins don't need to be compiled separately.

Users could supply their own repository of checks and there could even be a community repository of checks separate from the "core" plugins bundled in the main pint repository.

I'm happy to add more detail if needed but feel the links to Sloth's plugin system and Yaegi may be enough for @prymitive to get the idea :) I'm also willing to turn this into a Discussion instead if that is a better fit.