This PR implements basic inter-process communication (IPC), which is an API-only feature - not available through CLI.

Command has the following new fields/methods:

• send: to send messages
• messages.incoming: to listen for received messages
• messages.outgoing: to listen for sent messages

The caller must set the ipc property when calling concurrently(). It sets the number of the file descriptor on which to set up the IPC channel.

Example usage:

const { commands } = concurrently([{ command: 'echo foo', ipc: 3 }]);
commands[0].send({ hello: 'world' })
commands[0].messages.incoming.subscribe(({ message }) => {
	console.log('command 0 sent this:', message);
});

With this PR I also realise that the README is no longer a good option for documentation.
We need to expand it into a website...