Add ability to filter requests being logged

I had the need of being able to filter out noisy requests being made. I only wrote tests for `NetHTTP` so far, but if this is a feature you're willing to add to master I could write tests for the other adapters.

Two new options are added: `:url_whitelist_pattern` and  `:url_blacklist_pattern`. Both options should be regex patterns. If a whitelist pattern is set, the URL of the outgoing request must match the pattern if it is to be logged. If a blacklist pattern is set, the URL of the outgoing request must *not* match the pattern. Blacklist pattern overrides the whitelist pattern.