Add ecma-262 number formatting option

[exjesper]

ecma-262 (ES6) is commonly used with json and has specific rules for NumberToString formatting. This PR adds an option to have ES6 like number formatting for the internal dtoa implementation. See: http://www.ecma-international.org/ecma-262/10.0/index.html#sec-tostring-applied-to-the-number-type

Inspired by: https://github.com/golang/go/commit/92b3e3651dc44f54b458f171f641779f10fbaec0