maximum encode_number_precision value

[rexxar-liang]

for the latest code， I found the maximum value of encode_number_precision is 16, which is merged in commit.
openresty@f79aa68

For our scenes， we use int64 value in json strings, which means 16 precission numbers is not enough for us.

So any suggestions?
Can we change max value of encode_number_precision to 19, 20 or add new configuration like cjson.max_encode_number_precision(X) for different scenes?

Thanks.