Different ways of providing the invoice macaroon

[sputn1ck]

The invoice macaroon currently needs to be provided in the webhook url. This is not that great, as github has a limit of characters that can be set as a url (I ran into this when the url was "github.donnerlab.com").

We could add the macaroon in the repo itself, however i don't think that should be publicly accessible.

Open to suggestions

[Kixunil]

I'd definitely be interested in invoice being provided in a configuration file. I'd love to package this app in my repository