Add dragonbox to compute the required precision to print floating point
numbers. This avoids uglification of floating point numbers that
happen by default via std::stringstream.
Numbers like 34.34 used to be converted to '34.340000000000003' as strings.
With this version they will be converted to the string '34.34'.
This fixes issue https://github.com/jbeder/yaml-cpp/issues/1289
