Adding RS-256/512 JWT signature methods

[phillip-elm]

I see the default (and only) option with this library is HS-256.

That's a perfectly viable option, but I'm curious to know if you've considered adding support RS-256 or RS-512 key pairs?

I really like the simplicity this library affords; seeing these added would be great.

[hernandev]

@phillip-elm Number 1 priority right now.