WISH: Support for specifying multiple token alternatives

[HenrikBengtsson]

(This used to be part of #38)

We currently have in ucsf-vpn (>= 5.3.0):

$ ucsf vpn start
Enter 'push' (default), 'phone', 'sms', a 6 or 7 digit token, or press your YubiKey: ...

Feature Request

Also, when prespecifying the token, it would be neat to be able to specify more than one, e.g.

$ export UCSF_VPN_TOKEN="push,digits,yubikey"
$ ucsf vpn start
Enter 'push' (default), a 6 or 7 digit token, or press your YubiKey: 

or

$ export UCSF_VPN_TOKEN="phone,push"
$ ucsf vpn start
Enter 'phone' (default), or 'push':