* Use require options as a list, instead of booleans

Deprecate the use of the boolean options

* Add test for the new require option

* Add documentation on how to use the require option

Co-authored-by: Pau Ruiz i Safont <psafont@ebi.ac.uk>

Co-authored-by: José Padilla <jpadilla@webapplicate.com>

* Add code example for `aud` being an array

The previous code example only showed the `aud` claim as a single
case-sensitive string, despite the documentation mentioning that the
`aud` claim can be an array of case-sensitive strings

Add a code block demonstrating the `aud` claim being an array of
case-sensitive strings to make it more clear to the user that it is a
permitted use of the `aud` claim

* Add example of the `audience` param as an iterable

Demonstrate to users reading the documentation that the `audience`
parameter is not restricted to the `string` type, but can also accept an
iterable, as implemented in PR#306

https://github.com/jpadilla/pyjwt/pull/306

* Fix short title underlines

Short title underlines throw warnings in reStructuredText linters

* Added header option for encode

* added tests and None check

* updated quotes for consistency

* format fixes after tox