[request] Enable editing atoms

[joshuafcole]

If the previous request #2 or something analogous to it goes through, it would be awesome if Birch could be used as a simplistic UI for exploratory coding. In my component entity system example, I'm doing a lot of evaling code to update various components. It's ridiculously cool and handy, but it would be even nicer if there were a persistent interface available for it that were more approachable to newcomers.

[mortalapeman]

The current implementation uses zippers and I know they support some sort of updating the entire structure. Theoretically it should be possible, I just haven't played with that part of ClojureScript's api just yet to envision the implementation.