Add version as a parameter.

[jdeisenberg]

I've added :cljs-version as a key in mies.clj; this should make updates easier. I've also added .repl-{{cljs-version}} to .gitignore and the :clean-targets vector. Finally, I added the target directory to the :clean-targets directory, though I'm not sure if that is a correct thing to do.

[swannodette]

I don't want to add any options or support any customization. This is a minimal thing.