Make JSON lib configurable

[abitdodgy]

New versions of Phoenix and Ecto are using Jason as the default JSON library. Any interest in making the JSON library configurable? Something like...

config :cloudex, :json_library, Jason

Then, whenever we are calling it, we can do something like...

@json_library Application.get_evn(:cloudex, :json_library)

def foo do
  @json_library.decode!(...)
end

[smeevil]

Sounds like a good plan 👍 otherwise making Jason the default would be fine as well.

[abitdodgy]

@smeevil see #58

[abitdodgy]

Merged in #58