Add `ignore_user_agents` configuration option

This allows users to specify an array of Regexps to ignore specified user agents, which we found helpful because of certain badly-written crawlers scraping our site with mangled query params.

I wasn't totally happy with the extra call to `Notification#generate_meta_data`, duplicating the one in the `#deliver` method, but it seemed preferable to checking both the `@meta_data` and `@overrides` ivars for the same thing, and I didn't want to do more extensive messing about. What do you think?

Cheers,
Simon