Fix release and prerelease behaviour for GitHub Atom in cask-appcast
Fix release and prerelease behaviour for GitHub Atom in cask-appcast

Show latest version instead of prerelease if its more superior when stability is set to 'unstable'. Also improved version labels, custom version comparison and comments.

victorpopkov/cask-tools,victorpopkov/cask-tools,victorpopkov/cask-scripts,victorpopkov/cask-scripts