Support strings in add and subtract

[ghost]

This adds support for strings on add and subtract.

moment().add('1', 'h');
moment().subtract('1', 'h');
moment().add('h', '1');
moment().subtract('h', '1');

Notice that the value is a string. At the moment, doing the add would not do anything.

[ichernev]

Well, the issue is that the initial interface was ('h', 1), and then we started supporting (1, 'h'), so we used the string type to distinguish the caller. We have to drop support for ('h', 1) in the next major.

The change looks good, though. Thank you @seriousben