Ability to print ints as hex

[cosmikwolf]

Somewhat related to #358 , but also separate.

I use ArduinoJson to serialize data before so it can be written to flash memory on a teensy based project. I have very limited memory, and if I could have a .set(int, HEX) to print to hex, and an inverse operation to read an int in by its HEX value, you could save significant space.

[bblanchon]

Hello Tenkai,
I'm not sure it's a good idea since it not standard.
This would generate a JSON that cannot be read by other parsers.