Feature Request: Tags

[catzAreVeryCute]

I think it'd be very helpful if there was a way to add tags to certain pages. For example, let's say I'm taking notes on DFS, BFS, graph algorithms, tree algorithms, and other search algorithms. DFS and BFS could be placed under search algorithms, but these algorithms can also be used in the graph / tree context. I'd like to tag DFS with "search", "binary tree", and "graphs." Then I could search via tags and filter all my pages by the tags, so if I want search algorithms that work with trees, I can search using the tags "search" and "binary tree" instead of having to look through all my pages.