Support ES256K (secp256k1)

[youngjoon-lee]

Hi all,

I'm trying to use this library for my platform, but my platform has been using ES256K keys, not ES256.

Do you have any plan to support ES256K? It seems there have been no discussions about ES256K in this repo.
If you think it's reasonable, I can open a PR :)

Please consider and let me know if I can do it.

[youngjoon-lee]

@lhazlewood Could you review this issue?

[youngjoon-lee]

@lhazlewood Hi. Could you review this PR?

[lhazlewood]

I don't understand the purpose for this PR - why did you create an alternative name for ES256?

The JJWT team will only support specification-defined algorithms, not custom ones. When #178 is complete however, you'll be able to add custom algorithms in your own project, so that might solve this use case? Please advise.