Add Token refresh endpoint

[qazsero]

Adds an Endpoint that gets a valid JWT through the headers and generates a new JWT with an updated expiry date

[gordielachance]

Please add it to the documentation ! thanks.

[qazsero]

Good point. I can do that later. Thx @gordielachance

[qazsero]

I can't create a PR for the wiki, so here's my fork https://github.com/qazsero/simple-jwt-authentication/wiki/Documentation