WIP! - Enable copying of citation to clipboard

[LukasWallrich]

I often find myself creating a citation, just to then find the file, copy it and paste it into - e.g. - PowerPoint. It would be convenient if this step could be skipped. This PR implements that, but is only tested on Mac ... the implementations on Windows and Linux might work, or they might not ...

If you prefer not to add this to the package, I fully understand that - it might cause some maintenance hassle. Otherwise, I'd be happy to develop it further based on any feedback you might have ... and will certainly use it in my own workflow.

(Note that this also includes the code in #62 ... I wanted them on my same branch to start using them both, but wanted to give you the option to deal with them separately ...)

[nucleic-acid]

That's a very good idea! I hope to find some time to merge it soon.