Interactive demo

Here's an idea I had while reviewing #416: It could be pretty useful to provide interactive demos for each of the methods in Globalize. Have a text input field for the value to format or parse or validate, a select or similar for the Globalize method, and another one for the locale. Then the user can pick, for example, `dateFormatter`, provide a date (using a datepicker?) and pick a locale to have that date formatted in the given locale. In this case it should use a default pattern, but also allow the user to pick something else.

Could provide this as a demo in this repo for a start, then later host it on `globalizejs.com`. Might fit into GSoC proposals.