Enable dynamic caching

[nilsnolde]

With #19 hitting soon and implement static caching, closing #4 , it'd be really cool to have a dynamic way of creating & flushing a new cache.

@danilopinotti had the great idea to use webhooks from Github/Gitlab/Bitbucket (with authentication?) to achieve that. The effort is minor and mostly around parsing the POST content of the provider's webhook.

Note, static caching will always remain the default as it doesn't require any additional action on the user's side (like creating webhooks).