Doesn't fully support actual ISO8601 Duration format

Duration has years months, days, hours, minutes, seconds.  So why not support all of these?

```
irb(main):004:0>  d = Duration.new('P3Y6M4DT12H30M5S')
=> #<Duration:0x007fdb4ea67df0 @seconds=5, @negative=false, @total=110766605, @weeks=183, @days=1, @hours=0, @minutes=30>
irb(main):005:0> d.iso8601
=> "P1282DT30M5S"
```

Shouldn't the output return the same format as the input?