This allows registered plugins to execute code when loaded. This will
also allow the Lua support code to be a plugin.

Closes: https://todo.sr.ht/~sircmpwn/koushin/54

This allows to access the request metadata and the session from injectors.

This allows Go plugins to easily inject global data.

References: https://todo.sr.ht/~sircmpwn/koushin/39

This function doesn't render anything, it just adds extra data to the
template data.

References: https://todo.sr.ht/~sircmpwn/koushin/6

This will allow SessionManager to re-connect when the IMAP server logs
the user out.