Update Contributing guide

* Update Alfred.app to the latest spec
* Add Vagrant.pkg to illustrate the difference in Casks
* Add `install` and `uninstall` fields to table
Add Backblaze Downloader Cask
Merge pull request #842 from jgarber623/backblaze-downloader

Add Backblaze Downloader Cask
Update MacUpdate Desktop to 5.2.3
Merge pull request #843 from NanoXD/macupdate

Update MacUpdate Desktop to 5.2.3