Implement buffer completer

[cole-h]

I noticed that there is no buffer completer implemented yet -- is this because of the space menu's buffer picker? Would a buffer completer be accepted if I hypothetically contributed it some time in the future?

Originally posted by @cole-h in #1035 (comment)

That would be great! I think we had no commands that took a buffer before. I also wasn't sure, should we allow specifying buffers by path, or by an ID (or both)? After e2560f4 it's now possible to get the actual numeric ID, and we could display it in the buffer picker too.