Add --raw flag to send and receive commands

[mellonnen]

The rich terminal UI is nice, but it makes it impossible to use portal in scripts. We should provide a flag/config for running portal without the rich UI

For the sender, it could look like this.

> portal send README.md
 1-comet-spin-system

And for the receiver, it would simply output nothing. On errors, we could write the errors to stderr.