Add an inject function, that can be used by plugins that want to replace
one of ikiwiki's functions with their own version. (This is a scary thing
that grubs through the symbol table, and replaces all exported occurances
of a function with the injected version.)
external: RPC functions can be injected to replace exported functions.
Removed the stupid displaytime hook, and use injection instead.
Need to use a hook because an exported function cannot be reliably
overridden. The replacement verstion was actually only affecting plugins
loaded after it.
formattime doesn't need a hook, since there's no reason to export it.
Joey Hess