Move rules out of this extension repo

[floehopper]

It's less obvious that the rules should live here now that this extension is more generic.

[floehopper]

We should probably still keep some kind of example rules.json in this repo.

[tomstuart]

It’d make sense for an example rules.json to (also?) be in a gist somewhere, so that people can easily fork and customise it. (FWIW, the first thing I did was run it through a JSON pretty-printer so that it was nicely indented.)