Custom unit scalars

[brawaru]

After weeks the definitions of units becomes loose, since they are not fixed: month can be mean 28 days in financial sphere, or they can mean 30 or 31 days. Years sometimes can be 366 days.

Intl.RelativeTimeFormat does not care about that, it's your responsibility to provide the correct unit. However, we have our pre-defined units, so it would be great to be able to configure them to account for these non-standard cases.