Allow using promise as first argument

[jcubic]

I have an idea for a new feature for jQuery Terminal

This will simplify the code when using commands that require RPC:

const commands = rpc({
    url: 'https://terminal.jcubic.pl/service.php',
    promisify: true
}).then(service => {
   return {
      something() {
        return service.something();
      }
   };
});

$('body').terminal(commands);

The terminal will be created instantly but waiting for the RPC to init.

ref: JSON-RPC lib.