save the window contents to a file

It would nice to have a `save` fn that stores the screen contents to a file.

Trying to require quil.core and use the [image output API](http://quil.info/api/output/image) wasn't sufficient. Or perhaps I'm missing something.

If Quil's `save` fns only work within sketch fns, it may mean clojure-turtle would need to wrap that behavior such that `save` only executes once, not every time Quil draws a new frame (ex: 30 or 60 times / second).

If this functionality is feasible, it should be feasible in Java Clojure. It may be possible in CLJS too, but I'm not sure what the differences would be between the standard REPL from a CLJ-bundled Jar and a npm-based CLJS REPL.