Backport mod_substitute from trunk:

mod_substitute: Added a new experimental output filter, which
performs inline response content pattern matching (including
regex) and substitution.

Submitted by: jim
Reviewed by: jim, rpluem, fuankg


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@602684 13f79535-47bb-0310-9956-ffa450edef68