Add a new option decimal_as/1 to json_read to allow reading numbers w…

[thetrime]

…ithout any loss of precision. See issue #130

This isn't especially elegant, but it's simple and seems to do the job!

[JanWielemaker]

Hi Matt, Kind of missed/forgot this. I guess this is still something we want, although the current rational support may ask for a somewhat different interface.