Improve user experience for input field / keydown event listener

[daniel-j-h]

Right now we add a keydown event listener, so we can act on the "botton down" event already - which feels more responsive than waiting for the "button up" event. The downside is, we have to re-implement special keys such as backspace and remove the last character from the right.

postcodes/docs/index.html

Lines 49 to 68 in e81a7f7

	postcode.addEventListener("keydown", function (e) {
	var code;
	// We want to fitBounds when the key is down already;
	// this means we have to handle special keys such as
	// backspace manually and delete the last character.
	// Maybe there is a better way, because this e.g.
	// doesn't handle text copy & paste or cut events.
	if (e.keyCode === 8) { // backspace
	code = postcode.value.slice(0, Math.max(0, postcode.value.length - 1));
	} else {
	var digit = parseInt(e.key, 10);
	if (isNaN(digit) || postcode.value.length >= 5) {
	return;
	}
	code = postcode.value.concat(digit);
	}

This is pretty hacky, does not work with composite keys (think ctrl+x), and it does not seem to work properly on Firefox for Android.

We should look into improving the user experience around the input field.