[New] SingleDatePickerInputController: Allow passing multiple date formats to make validation more permissive

[Floriferous]

Closes #345.

This is a proposal at the moment, I didn't yet look at all the other places this needs attention for. So I'd like to get some feedback before going in deeper.

The basic idea is this:

<SingleDatePicker
  dateFormat={["D MMMM YYYY", "D/M/YYYY", "D.M.YYYY"]}
/>

The first format in the array will be used to display dates, and all the other ones will be used to validate it while typing, to improve the user experience, as fewer inputs will be rejected (there is also no error shown, leaving users stranded).

The main issue with this pattern, is that you can provide conflicting date formats, such as DD/MM/YYYY and MM/DD/YYYY, in which case it's unclear what will happen. I could try to write a helper to throw when this happens, but documentation might be enough.

However I believe the benefit outweighs the potential misuse, as just my example above with a couple more formats will help a lot!

[ljharb]

Thanks! We'll need a documentation update, and lots of tests to cover this addition.

[ljharb]

Suggested change

	if (typeof displayFormat === 'string') {
	return displayFormat;
	} else if (Array.isArray(displayFormat)) {
	return displayFormat[0];
	}
	return displayFormat();
	return typeof displayFormat === 'function' ? displayFormat() : [].concat(displayFormat)[0];

[ljharb]

Suggested change

	return displayFormat;
	return [].concat(displayFormat);

this way it's always an array

[ljharb]

Suggested change

	let dateFormats = [];
	if (Array.isArray(customFormat)) {
	dateFormats = [...customFormat, DISPLAY_FORMAT, ISO_FORMAT]
	} else if (customFormat) {
	dateFormats = [customFormat, DISPLAY_FORMAT, ISO_FORMAT]
	} else {
	dateFormats = [DISPLAY_FORMAT, ISO_FORMAT]
	}
	const dateFormats = [].concat(
	customFormat || [],
	DISPLAY_FORMAT,
	ISO_FORMAT,
	]);