Refactor Nikola.path

[ralsina]

Currently Nikola uses Nikola.path() and Nikola.link() to resolve a number of things like "what's the path to the tag called foo".

The problem is that these are in the Nikola class, which therefore needs to know a lot about the internal workings of a bunch of plugins, which means the separation of concerns sucks.

Solution, make plugins register their own path resolution mechanisms with Nikola and then use them.

[ralsina]

@DBeath already did a large chunk of this in #814