Make Basic Auth realm filterable

I've been running into some frustration using basic auth on multisite installations because each network site uses a different Realm value and has to be authenticated to separately.

https://github.com/humanmade/PHPBasicAuth/blob/master/inc/namespace.php#L146

I _think_ that it's possible for different domains to share basic auth state if they use the same realm. If this is possible, it would be nice to either (a) use a single authentication realm for a network by default, or (b) wrap the realm value in an `apply_filters` call, and allow sites to define their own logic for enabling authentication. (or both)

If this is not possible, then this issue can be closed as invalid.

Relevant documentation: https://tools.ietf.org/html/rfc2617