Add plugin interface to allow editing and extending of config by extensions

[rgrp]

Use case: extensions need to add to template and public search paths if they wish to have templates or extra static files (such as js etc).

Allowing plugins to edit the config dictionary on load would allow this (and more!).

Are there any security considerations here? (Is is bad to allow extensions to make arbitrary changes to the config information?)

[rgrp]

Closed by cset:83734b5e251c which implements an IConfigurer interface.