OSX support

[dgsuarez]

We're using xclip to copy the contents. On mac we have pbcopy, however it seems that there's no direct way to set the mime type of the copied data and it's always copied as if it were text.

This stackoverflow/superuser answers have some ideas, but I don't currently have a Mac to test them out:

• https://superuser.com/questions/1132777/copy-an-image-to-clipboard-from-the-mac-terminal
• https://stackoverflow.com/questions/6919403/set-clipboard-to-image-pbcopy

If possible I'd like to avoid including a binary executable in the plugin.