Introduce hook_civicrm_alterContent

CiviCRM 4.x introduced the requirement that when utilizing CiviCRM hooks with Joomla that the code be "inserted" via Joomla Plugins. However, Joomla Plugins cannot manage files in the CiviCRM Custom directories. This prevents hooks from directly taking advantage of the template_name.extra.tpl feature which was also introduced.

In an effort to alleviate the need to manage files outside of the plugins folder the attached patch creates a hook that receives the entire content of the generated page after all template.extra.tpl files have been "attached". The hook is then free to modify the content. In an effort to provide some idea of the context of the call the hook also receives the name of the template being utilized.