Pug Linker - Plugins

[ForbesLindesay]

I'm not that confident what a plugin API for the linker should look like, but my current thinking is to have the following hooks:

1. pre-link (gets given asts that are about to be linked)
2. pre-extend (gets given the parent ast and the child ast before any blocks have been resolved)
3. extend (gets given the parent ast and child ast before any blocks have been resolved, but then skips built in block resolving)
4. post-extend (gets given the parent ast and the child ast after blocks have been resolved)
5. pre-include (gets include node before yield has been applied)
6. include (gets include node before yield has been applied, but skips applying yield)
7. post-include (gets include after yield has been applied)
8. post-link (gets given asts after all other linking is done)

We should handle raw/filtered includes somewhere else so they don't feature in this proposed plugin API.