extensions: support callbacks after another extension loads
An upcoming patch will introduce a dependency between the color and
pager extensions. To prepare for this, we teach extensions how to
register callbacks that can execute when another extension loads.
This patch is based on code provided by Matt Mackall. But significant
parts have changed (such as the ability to register multiple callbacks
and the change in behavior to always call a callback).
I believe that always firing the callback is a good practice. I think
the common use for this feature will be for extensions to say "run this
one-time setup code, after this other extension if possible." Always
running the callback will facilitate this.