Change feed action hooks to filters.

[majelbstoat]

At the moment, we can't edit rss or atom head information in feeds, just add to it. It would be better to change rss_head, rss2_head, atom_head (and probably the others too) to filter hooks passing the information that has already been presented so we can preg_replace or str_replace it.

The specific case I'm trying to solve is editing the language entry in the feed head section. At the moment, the only way to do that is to alter the database rss_language option every time a feed is called, which is unfeasible.

The incidental changes made in #3638 are a step in the right direction and allow plugins to add new feedtypes or replace their handling with a new plugin-defined function . However, plugins still cannot make small alterations to feeds such as changing the title or the language, without having to rewrite the entire feed output function.

You can modify options like rss_language on the fly usign the hook inside of get_settings.