JSON encoding for 64 bit numbers.

[samuelvarley]

Google's spec for converting between proto3 and JSON[1] specifies that 64 bit integers should be encoded as JSON strings instead of JSON numbers. I think it would be a good idea to follow that because:

• Javascript cannot represent all 64 bit integers in its native numerical type.
• It would allow the full range of uint64 to be used.

However, the spec also states that 64 bit integers encoded as JSON numbers should be accepted (i.e. be permissive). Following that may not be a good idea because it could appear to work when testing with small integers but fail with larger integers that Jansson cannot handle.

Would you be interested in a pull request for this? Do you have any other insight I should be aware of?

[1] https://developers.google.com/protocol-buffers/docs/proto3#json