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"`
Could this just be renamed `TimeSpanFormat` rather than having the new interface? I haven't looked at this in a while, how would the interface be used?

as:

    public class TimeSpanHumanize
    {
      public TimeSpanHumanize(ITimeSpanFormatProvider timeSpanFormat) { }
    }

or:

    public class TimeSpanHumanize : ITimeSpanFormatProvider { }
    public class TimeSpanShortHumanize : ITimeSpanFormatProvider { }