Support promise, async/await for some important actions like sign, verify jwt.

[mattnguyengit]

Describe the problem you'd like to have solved

I usually use sign, verify function for my authentication process by synchronous syntax. I don't know the process is heavy or not but better to use asynchronous for it.

In documentation, I see we have callback syntax but it is not popular on modern code these days.

[panva]

Duplicate of #111