Add sourceRegex option to extract source names from measure names.

This is similar to https://github.com/librato/statsd-librato-backend/pull/8#commits-pushed-82e5e42 but uses a regex to allow for arbitrary source/measurement name formats.

Ideally statsd would let us do name validation in the backend, but this is still useful until then, just with a limited set of characters.

For issue #2