Feature Request: Support plaintext payload

[ignassew]

Currently when using jwt-cli with a payload that is not JSON, we'll get an error.
According to RFC7515 Section 3.3:

Note that the payload can be any content and need not be a representation of a JSON object.

I can try to write the code for it when I have the time.

[mike-engel]

Hey @ignassew, can you give me an example use case? This is not a trivial thing to add, since to much of the code is dependent on key/value pairings. Thanks!