Allow to pass the precision for float numbers

Currently sass process files with default precision of 5 digits if I'm not wrong
In bootstrap the base-line-height has 9 digits of decimals which cause a problem when the sass precision is less that 9
So I think this is the best way to add the precision we need
Merge pull request #674 from trivago-makbarof/patch-1

Allow to pass the precision for float numbers