Adding a cask for the mail plugin MailFollowUp

The installer comes with an uninstaller app (an Automator Workflow at
heart), but I wasn't able to run that with 'script' so I set the
uninstall to manually delete the plugin folder.
correct various issues with the slack formula
Merge pull request #2116 from taylorlapeyre/master

Correct various issues with the slack formula
corrected some namings, according to conventions
Merge pull request #2118 from vitorgalvao/correct-some-namings

corrected some namings, according to conventions
Merge pull request #2080 from lowbatteries/mailfollowup

Adding a cask for the mail plugin MailFollowUp
Merge remote-tracking branch 'phinze/homebrew-cask/master'