add 'round' option for fontforge engine

As documented here
http://fontforge.org/python.html#cnt-round
round is useful to limit the output size of your fonts.

For example, to round to 2 decimal places, set `round: 100`.
To round to 1 decimal place, set `round: 10`.

Setting `round: true` will be interpreted as `round: 1` (0 decimal places).

I don't think rounding should be coupled to `normalize` either, so I uncoupled it.