Support more properties of Sparkle in appcast

[oscarmarcelo]

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.