Added ability to get short time identifiers for TimeSpan

I needed the ability to get "ms", "s", "h", etc time identifiers.

For example, `TimeSpan.FromHours(2).Humanize()` I'd like to be able to have `"2 days"`or  `"2d"`
The SI system does not add 's'. I.e. 1 minute -> '1m' and 2 minutes -> 2m