Support directory transfer

[abitrolly]

Nice app. I was able to transfer files to magic-wormhole. The only problem is sending multiple files as it requires selecting each file separately and to type new code for each.

Maybe it is possible to extend the interface to select directory and/or multiple files to send with https://github.com/magic-wormhole/magic-wormhole/blob/master/docs/file-transfer-protocol.md#sender