Feature: Extension system for application-specific usage checking

[TSMMark]

Continuation of #48

add some sort of extension system. I'd love to extract the rails code out and the graphql stuff makes sense elsewhere

• [ ] Implement a plugin system (maybe similar to Jeremy Evans plugin pattern, described here https://janko.io/the-plugin-system-of-sequel-and-roda/).
• [ ] Extract the rails logic into a plugin
• [ ] Extract the graphql logic from #48 into a plugin
• [ ] Document