Add `logout` or `revoke` command

[waits]

There's currently no way to log out unless you know where the auth token is stashed and go delete it manually. A simple logout/revoke command would need to do two things:

1. Call https://api.dropboxapi.com/2/auth/token/revoke to revoke the active token
2. Delete ~/.config/dbxcli/auth.json

[arafatx]

Oh my ... spending more than 3 hours looking for reauthorization, finally that is the directory it stored. should have put this in the main documentation in Beginner section !