Add API to use custom regexes.yaml file

[bitc]

This library currently embeds a version of regexes.yaml from uap-core and always uses it.

I propose to allow users to load a custom version of regexes.yaml. This way users can keep up to date with the upstream regexes.yaml without having to wait for new releases of this library, and users can add their own custom regexes.

The UAConfig type should be publicly exposed (including it's FromJSON instance), and then additional parse* functions should be provided that accept a UAConfig value.

I imagine that this won't be a lot of work to do, I can work on a pull request