This filter can rewrite URLs using regular expressions and back
substititions.  Rewrite rules are stored in a text file with one rule
per line.  Each line consists of a regular expression, whitespace, and
a rewrite substitution pattern.  Example rules:

    (^http://)www.hosta.com(/.*)	$1www.hostb.com$2
    /\?(http://.*)$			$1
    redir\?(http://.*)$			$1
    xfer\?url=(http://.*)$		$1

User preferences:

* Rewrite.rules

  Location of the rewrite rules file.  The default is ~/Muffin/rewrite.
