Add decimal place support

[almandsky]

As per https://github.com/zeit/ms/issues/124

Here is the PR to add decimal place support. Default is 0, max decimal place is 3.

ms(60000, { decimal: 1 })                           // "1m"
ms(66000, { decimal: 1 })                           // "1.1m"
ms(-3 * 66000, { long: true, decimal: 1 })          // "-3.3 minutes"
ms(234234234, { decimal: 2  })                      // "2.71m"
ms(ms('10.5 hours'), { long: true, decimal: 1 })    // "10.5 hours"

[mahnunchik]

ping

[shwarcu]

@leerob @gmattie any chance for having this functionality added?