Fix #896 -- support hooks in templates.

This is #896.

Currently untested.  Requires some.  The list of hooks, as of now, is very small: `extra_head`, `body_end`, `page_header`, `menu`, `menu_alt`, `page_footer`.  There are many more that require inclusion — though I couldn’t think of many right now — this requires work.

cc @Aeyoun (#896 creator)