Add Format API, `Measure.prototype.toString` may not be enough

[tpoisseau]

What about a format API interoperable with Intl API design ?

see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat
see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString

Provide an Intl.MeasureFormat API and Measure.prototype.toLocaleString shortcut.