Add support for startOf("decade")

[lburch02]

I find the lack of a startOf("decade") function frustrating because there is no simple way to get the date of the start of a decade.

The same way that moment().startOf("week") works and returns the first day of the week, I would like a moment().startOf("decade") which returns the first day of the decade.

[cutterbl]

If I were to extend moment (plugin), how could I define a 'decade' or a 'century' so that math would work out of the box (add, subtract, startOf, etc). It would seem that I would start with defining a Duration somehow, and I know I would have to extend my prototype to include the proper methods (decade(value?)). Just trying to wrap my head around how I would go about this...