Support more properties of Sparkle in appcast
Currently, the only supported properties related to plugin updates seems to be `.appcast.xml`'s `<enclosure>` and `manifest.json`'s `homepage`, in which the latest is used in both '*View "\<Plugin\>" Documentation*' in the context menu, and when the plugin has an update.

It would be nice if the the plugin update link ('*Version <x.x.x> available*') used Sparkle's `<sparkle:releaseNotesLink>` instead of `homepage`.

Additionally, `<title>` and `<description>` would be interesting to use, giving the user the option to read the release notes without leaving Sketch.