support async (promised) validators

[icebob]

Handle async validator which returns with Promise. Discussion: #170

Example async validators in dev:

let customAsyncValidator = function(value) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			if (value)
				resolve();
			else
				resolve([ "Invalid value from async validator" ]);
		}, 1000);
	});
};

Schema:

{
	type: "input",
	inputType: "url",
	label: "URL",
	model: "website",
	validator: customAsyncValidator
}

TODO:

• add tests

[cristijora]

Nice. @jmverges take a look :)

[jmverges]

@icebob works properly for me, thank you!

[icebob]

Great! You're welcome.

[icebob]

Released in v2.0.0-beta6