Add support for cookie 'samesite' option

[farshiana]

No description provided.

[coveralls]

Coverage remained the same at 100.0% when pulling 7a33f9c on farshiana:option_cookie_samesite into 32a1f3c on vimalloc:master.

[vimalloc]

This is great. Thanks for contributing! 👍

[johaven]

At this time latest version of Flask does not require Werkzeug 0.14 but 0.13.
I got this error with 3.6.0 when set_cookie is called:

flask_jwt_extended/utils.py", line 198, in set_access_cookies
    samesite=config.cookie_samesite)
TypeError: set_cookie() got an unexpected keyword argument 'samesite'

It would be cool to keep a compatibility mode :)

[vimalloc]

It looks like the requirements for flask are actually just Werkzeug>=0.7, so on a fresh install of flask it should have a sufficiently high version (as of writing 0.14.1), but if you already have flask installed it may be out of date. That could be fixed by doing an upgrade of Werkzeug manually in your virtual environment.

I'm going to add Werkzeug>=0.14 to the requirements of this extension to help alleviate this. If this is not sufficient to solve this issue I will look at ways to disable the samesite feature on older versions of Werkzeug.

Cheers :)

[johaven]

You have right, latest version of Flask requires >=0.7, when i installed the last Flask version the version 0.14 was not out. Thank you :)

[johaven]

Another little request, in documentation could you specify for JWT_COOKIE_SAMESITE section :

Possible values for the flag are lax or strict