Add authentication support for custom webhooks

[cryptaliagy]

After closing #11, basic auth & bearer auth can now be more safely implemented, as there is an alternative to explicitly setting this information in the configuration file.

These should be provided under an auth key for the webhook, which should support either option depending on which subkeys are used (username/password for basic auth, bearer for bearer auth).

I think it would also be a good idea to emit a warning if basic auth is used to discourage the use of it.