Add gpg stanza to compatible casks

WIP — **do not merge** yet.

Add `gpg` stanza to casks for which signatures and keys are available.

(A note on cask layout: as they serve similar purposes, `gpg` and `sha256` are grouped together; we might want to change that.)

Known compatible:  
- [x] cryptol
- [x] gpgtools
- [x] torbrowser
- [ ] libreoffice