Show link alias instead of Zettel title when available

[voidus]

When writing [[xyz|foo]], I'd expect it to show foo instead of the zettel title.

[pyrho]

I've added this feature in the feat/title-alias branch, you can check it out if you'd like.

I would like to have proper unit tests on the whole virtual title bit before merging into master (this is currently wildly untested, and I wouldn't want to break things for the two other people out there relying on this :P