Support promise returning commands

[evaera]

No description provided.

[autonordev]

A complication with this is that the most common Promise implementation has nothing necessarily unique about it, so if a table is returned we have to decide between treating it as a problem or if it 'looks' like a promise, then treating it as one.

We could do this by seeing if it has both a resolve and reject functions. If so, we'll use these; I don't see much risk with this.

When a command returns a promise, resolve would be treated like usual command output but we need to decide between treating reject like:

1. an internal Cmdr error (red text in the output with the message)
2. regular output (white text in the output with the message)
3. like any other command error (red text in the output and warning emitted)

Personally I lean towards 3 since it's not our job to handle errors for developers, but we can be kind and prevent obscure unhandledPromiseRejection warnings.