Feature Request: Run custom, user-defined rules

[radiovisual]

If there is a feature you think everyone will need, then please add it to this CLI for everyone to benefit from, but sometimes there will be custom needs of a specific project where you want to make additional checks that are too specific to your needs and should not be part of this repo. For those scenarios I propose a way to register these custom rules to be defined outside of this repo, and safely defined inside your own application.

The configuration could look like this:

{
  "customRulesDirectory": "path/to/custom/rules/dir"
}

And all the rules in that directory will be run in the same process as the official rules, and be expected to report their problems in the same way as real rules. For all intents and purposes the custom rules should be defined to work the same as the official rules, but they just live outside of this CLI.

This means that we will also need to write a step-by-step guide on how to write a custom rule. This will be nice to have anyway to help contributors write rules for the CLI, as well as help users of the CLI define custom rules that live outside of the CLI.