Document tags

I think tags would be a really useful addition to outline - is this something you have planned, or an idea how you want to implement it? I could potentially help getting it done.

One method that I think works quite nicely is how Bear (http://bear-writer.com) implements them - you just include a hashtag in a document and it counts as a tag. I've thrown together a very quick and hacky way to implement them in outline using search:

https://github.com/outline/outline/compare/master...mixcloud:hashtags

It uses the fact that postgres full text search thinks that //tag/... is a file and doesn't split it when it's indexing. Obviously this is not ideal! Ideally the tags would be clickable in the documents, and I would have created a new migration (since this is a hack I just wanted to quickly try it out without having to learn sequelize though).