ROUND_UP on flat_tax function

[jdivins]

Hello,
Could be possible add to the library an option to allow chose between round_up or round_down on the flat_tax function: https://github.com/mirumee/prices/blob/master/prices/tax.py#L30 ?
By this way:
gross = (base * fraction).quantize(Decimal('.01'), rounding=ROUND_UP)

And then allow choose this config in django-prices-vatlayes, here:
https://github.com/mirumee/django-prices-vatlayer/blob/master/django_prices_vatlayer/utils.py#L101 ?

I have tried with an amount of 171.50 * 1.21 of TAX
And the final result is = 207.515
With two decimals I expected that the final value has been: 207.52
But I get 207.51

Thank you!