Use JSON standards for parsing

[Skehmatics]

String -> native object conversion is hard and opinionated, but if there's one standard that is used the most widely, its JSON.

This changes flask-env to use python's JSON library to parse the environment variables instead of the existing code. This diverges from the old standard in regards to handling boolean and float values. See the changes to the test cases.

Feel free to close if you disagree with this way of handling things. I don't take it personally :)