Add JSON column support, and related functions

[ssandler]

Should only be enabled in 5.7 mode

https://dev.mysql.com/doc/refman/5.7/en/json.html

The column type is just treated as a string so that part's easy, but the functions are the interesting parts.

[lexidor]

I'll take a crack at the functions. @someone_else can take a look at how to stitch them into the project.
I have not been able to implement the json functions. JSON_DEPTH does not agree with php's json depth in json_decode.