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.
Merge pull request #185 from mstorus/fontforge-round-option

Add 'round' option for fontforge engine.