Allow additional parameters to ab

For all the sites that I wish to test it is necessary to add additional parameters to ab. For example, add a cookie so that Drupal pages are served instead of Boost cached pages:
-CDRUPAL_UID=1 http://example.com/
Or if the site is behind an HTTP password:
-Auser:pass http://example.com/

This patch allows this. It does some regex work to prevent harmful characters. However I can't seem to find a definitive list of what characters should be disallowed. So currently this only allows a very strict set of characters.

Eventually these validators should be moved out into seperate functions so that they can be used to validate the original form input as well.