add platform-friendly copy and paste

While writing the [gitbeam plugin](https://github.com/cldwalker/gitbeam), I was unaware this was possible with node-webkit. Hopefully, this makes it easier to use the clipboard, for end-users or [plugins](https://github.com/cldwalker/gitbeam/blob/ae630679b4fded97cc91375ea1f3babc419835b1/src/lt/plugins/gitbeam/util.cljs#L80-L85). I noticed editor.cljs has a few references to clipboard. I can update those to use these fns if desired.