3rd Party Caveat Support

[bucko13]

The verification method on the macaroon class from js-macaroon accepts a third argument for discharges. Presumably this is for verification of 3rd party caveats. Currently the lsat-js library doesn't support this interface. There's not a clear need or use case for it at this point, but it's worth keeping in mind for future development.

discharges are used in the js-macaroon code here