Adds a path prefix and a handler for that path. If the path does not start with a / then one will be prepended.
The match is done on a prefix bases, so registering /foo will also match /foo/bar. Though exact path matches are taken into account before prefix path matches. So if an exact path match exists it's handler will be triggered.
If / is specified as the path then it will replace the default handler.
@param path If the request contains this prefix, run handler.
@param handler The handler which is activated upon match.
@return The resulting PathHandler after this path has been added to it.