Add option to use GPGTools with pass

The homebrew-provided version of GPG is quite difficult to configure,
and Homebrew Cask provides a nice option to install GPGTools (which
takes care of a lot of configuration). This modifies the pass formula to
optionally skip the gpg dependency, with the expectation that the user
will install GPGTools separately.
Instead use a `Requirement` to see if it's already in the path.