Support for Duration format

Thanks for a neat library.
I have tried more than handful of them and this is the one on by all criteria.
Would you consider this patch?

Added the style #3 for duration number format [h]:mm:ss
When the value of a cell is an object with the property 'v'
the generated cell uses the properties 's', 't' and 'v' for its
markup.

For example to output 1 day and half in a cell the value should be:
```
    { v: 1.5, t: 'n', s: 3}
```

That will be displayed as 36:00:00