Add recipe for GPGTools. `link` line intentionally ommitted.

I didn't know how to handle the `link` line for this cask because it can install:

 * more than one app (`GPG Keychain Access`, `MacGPG 2`)
 * items which are not apps (context menus, preference panes)

Each of the items in the install (
[GPGMail](https://gpgtools.org/gpgmail/index.html),
[GPG Keychain Access](https://gpgtools.org/keychain/index.html),
[GPGServices](https://gpgtools.org/gpgservices/index.html),
[GPGPreferences](https://gpgtools.org/gpgpreferences/index.html), and
[MacGPG2](https://gpgtools.org/macgpg2/index.html)) are distinct efforts
 with their own installers, so I will create separate casks for each of
them. However, I think that the combined offering,
[GPGTools](https://gpgtools.org/installer/index.html), should have a
cask as well.