Choose conversations to export

[ReagentX]

This came from feature request https://github.com/ReagentX/imessage-exporter/issues/51.

Right now, imessage-exporter exports every conversation it can see. It could be useful to specify specific conversation(s).

The easiest way to accomplish this is to know the chat's ID in the table, but even that may not be correct due to the duplication problem. We can of course show this to the user, but selecting from a list in a reasonable manner is an interesting UX problem.

I don't want to emit a huge list and expect the user to read through it, there must be a way to live-search it.