Additional configuration options #2

There's definitely some things which should be made configurable:

• Have a separate filter system for code fixers. Effectively we should fold over both linter and fixer lists, and take the intersection.

• Allow wildcard tag filters (i.e. var:unused*).

• Promoting/demoting warnings to a specific level. It might be nice to disambiguate between "this should cause a lint failure" and "this should be promoted to error".

• Specify a list of known globals, as well as standard libraries for them.