Add support to download a copy of a shared note into my vault

[jvsteiner]

I think there is a use case, for when there are two users and both have Share Note installed. As the recipient of a shared note, I would like to be able to download a copt of the shared note into my vault, and then maybe make some modification to it. Perhaps I will reshare it with the original author or maybe someone else. I realize there is already a feature request for editing and comments, but this is really different. It's less about providing edits and feedback, although, if I reshare to the original author, that is indeed what might be happening. Instead, this feature is more about the ability to pass a note to someone - who can then take it over, or use it for their own purposes.

I'm explicitly not suggesting that the note needs to be synchronized between all shared users. Although that sounds cool, I think it's much too complicated, and unnecessary for the kind of situation many people might have.

[jvsteiner]

I made a proof of concept plugin from a fork that can be used with BRAT here: https://github.com/jvsteiner/send-note