Allow for multiple instances of the Brain that can reference and automatically link to each other

[aengusmcmillin]

This is a bit of a longer term idea, but eventually it could be useful to explicitly support allowing this theme to be instantiated multiple times on a single site reading from different folders and writing to different urls. This may be possible now but it is untested.

And to pull the real power out of that, if possible the different instances should be able to optionally know about each other and automatically provide links to the same note on the other instances if it exists.

So for example, you could have a site shared between Bill and Jane with the following structure:

- content
-- bill_brain
--- books.md
--- movies.md
-- jane_brain
--- books.md

And when the books page is rendered for Bill it could include a link to Jane's books page, and vice versa, but nothing would change for Bill's movies page. I don't know exactly how this could be used, but it feels like there are continuations of this idea that could make it an incredibly powerful shared library of information.

Idea from here.