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.

首先检查 Quil 的图像输出 API 以及 issue 中引用的 sketch 函数。确定 save 是否可以只运行一次而不必在每个 frame 上重复运行，以及 Clojure 和 ClojureScript 之间的行为是否不同。当 save 函数能够在受支持的环境中可靠地将窗口内容写入文件时，即视为完成。

由索引模型根据 Issue 内容生成。