Add support for pushbullet "Link" types with config specified URL

This change will allow users to specify a jinja-formatted 'url' field in their config. If this field is specified, the pushbullet notification type will be a "link", allowing a user to click/press on the notification and be taken to the URL specified.

This is really handy, for example, on Android when you can specify an IMDB url, which will open in the IMDB app. You can then quickly view movie synopsis, view trailers, etc. In Chrome it'll open the IMDB page for the same purpose. Cool!