Add self-revoke cli command

[cblecker]

I'd like to request a top-level command in the vault CLI to hit the /v1/auth/token/revoke-self API endpoint and revoke the current token.

Thank you!

[michaelansel]

Having vault token-revoke without an argument "do the right thing" like vault token-lookup could be useful (albeit maybe a little risky). If you need a workaround, you can always vault write -f token/revoke-self.

[jefferai]

We specifically do not want vault token-revoke to "do the right thing" because it's too easy for that to be "oops, I hit Enter too early".

If someone wants to implement a -self flag I'd be fine with that. Send a PR if interested.