Add a leading 0 to the duration subparts

This way, 1:05 is displayed as 1:05 instead of 1:5
Wouldn't it be easier to use the format function instead? I think this has the same effect:
```coffeescript
class Durationformat extends Filter('common')
    constructor: (MOMENT) ->
        return (time) ->
            d = MOMENT.duration(time * 1000)
            m = MOMENT.utc(d.asMilliseconds())
            if d.hours()
                return m.format('HH:mm:ss')
            if d.minutes()
                return m.format('mm:ss')
            return "#{m.format('ss')s}"
```