hgweb: add websub template filter The purpose of this new filter is to make it possible to partially replace the functionality of the interhg extension. The idea is to be able to define regular expression based substitutions on a new "websub" config section. hgweb will then be able to apply these substitutions wherever the "websub" filter is used on a template. This first revision just adds the code necessary to load the websub expressions and adds the websub filter, but it does not add any calls to the websub filter itself on any of the templates. That will be done on the following revisions.