Feature Request: Send webhook on feed update

[wilt00]

I'd like to be able to trigger an HTTP request when a feed completes updating if there are new episodes available. Some use cases for this:

• Overcast has an API to trigger a feed refresh (This is the one I want)
• Podping is a similar (public-only) update mechanism, although I'm not sure what apps if any use it yet
• Sending notifications about new episodes through a messaging app like Discord or Slack

I'm happy to work on this myself, if you'd accept a PR for it. I'm not sure what an appropriate config setting for this would look like, though. I have a couple of ideas:

1. A new webhook option:

[feeds]
  [feeds.ID1]
    ...
    webhook = { method = "POST", url = "https://mywebsite.biz/ping", body = "{\"url\": \"$FEED_URL\"}" }

2. A more generic on_update option that runs a command or script when the update finishes:

[feeds]
  [feeds.ID1]
    ...
    on_update = "curl -X POST --data \"{\\\"url\\\": \\\"$FEED_URL\\\"}\" https://mywebsite.biz/ping"

This would allow more complex integrations, for example, as suggested in another recent issue, uploading new episodes to Archive.org using their API. Realistically, though, it would probably require users to build their own docker container to do anything nontrivial (I don't think curl is included in the alpine docker image, for one).

For both of these I'm envisioning some kind of template variable system where feed properties are exposed and can be inserted into the strings.

[ejj]

Do the maintainers have interest in accepting a PR for this? If so I would seriously consider implementing it.