Make Jason optional, allow replacing with Poison

[angelikatyborska]

For projects that already use Poison for JSONs, it might be annoying that this library requires Jason.

It should be possible to allow to choose either one or the other, like Phoenix currently does:
https://github.com/phoenixframework/phoenix/blob/51baf69993100c7ef40d097b02285ff28e4e16c9/lib/phoenix.ex#L69-L71
https://github.com/phoenixframework/phoenix/blob/2e5711c102a260a8d9457229c3dfd33f91c7cf99/lib/phoenix.ex#L87

However, that would make it slightly harder for everyone else to start using the library, because now everyone would have to explicitly add a JSON library as a dependency too (if it's not already there).

Are there any other popular JSON libraries that this one should support?

Please react with 👍 if you think that's a change worth adding.

[jc00ke]

This has maybe gotten a little nicer with Erlang 27 and the new :json module. IIRC Jason is going to use it by default if on Erlang >= 27, so maybe with a new version you can rev the requirements?