Support for less strict request matching

[lawnsea]

Production sites frequently make requests for which portions of the URL are generated dynamically and thus will not match the recorded URL in the HAR. For example, analytics scripts will often generate one or more unique IDs at runtime and include them in request URLs.

Currently, har-server.js matches requests based on strict equality by URL. It should be possible to relax this, possibly by specifying a regex or other pattern.