Add optional callback to config

[cosmicbuffalo]

I really like the handler logic built into this plugin, but can't use the open command easily over ssh. I was able to work around it by inserting a callback to stick the processed url into the clipboard when open is triggered. Figured this feature might be useful to others as well. A callback to insert to clipboard + osc52 + clipboard watching script achieves the same behavior over ssh as default gx on a local machine.