Enumerating rules

Related to:
https://github.com/jonase/kibit/issues/174

I just wanted to see myself where was this rule defined and if I could simply fix it maybe.

Differently from most other linting tools however the kibit report doesn't contain any code on each error, making it more complicated to know what it's referring to exactly.

I think it would be great to give a unique identifier to each rule and output that as well in the report.
This would also allow other cool things like filtering on certain rules, excluding checks and so on and so forth..

The issue names no files or tests. Start by locating the rule definitions and report-generation entry point, then determine how rule identity is represented and how reports are consumed. Done means each lint finding exposes a stable unique rule identifier; filtering and exclusions are suggested follow-on behavior rather than defined acceptance criteria.

Written by the indexing model from the issue text.