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.

Start by reviewing Quil's image output API and the sketch functions referenced in the issue. Determine whether save can run once without repeating on every frame, and whether the behavior differs between Clojure and ClojureScript. Done means a save function reliably writes the window contents to a file in the supported environment.

Written by the indexing model from the issue text.