Add support to multiple json libraries

[Quiwin]

Add support for multiple JSON coder using MultiJson

Hello again from Klaxit!
We need to be able to choose internally which JSON encoder is to be used, MultiJson is generally the most popular solution 🙂 .

[BuonOmo]

LGTM!

@keithdoggett I guess this could even go in a patch version since for people using JSON it won't change anything.
Maybe a minor just to warn users that if they are using any other json lib such as Oj they have a minor chance of encountering floating point issues (Oj < 3.10.4 ohler55/oj#583).

[keithdoggett]

Looks good to me as well.

@BuonOmo I agree that we could handle it as a patch, but I think a minor version is better because of the issue you pointed out and because a dependency is being added.

I also want to link these PRs #29 and #38 and explain why this isn't just a reversion to old behavior. There used to be an option to use different parsers and would require the parser at runtime. This was deprecated in favor of just using the standard parser.

The solution here is different because the APIstays the same and all of the parser selection is handled in multi_json which will use the standard parser by default.