Global index

[cilynx]

This will be useful for tag searching / filtering and is also the right way to ensure no (highly unlikely) UUID collisions.

[cilynx]

For the moment, with very few docs loaded, list comprehension around all of the UUIDs in the filesystem is doing the job. I'm going to stick with this terrible way of doing it as long as it works, mostly because I want to see how big the library gets before it gets too slow / memory intensive / whatever happens.