Add Copy OTP action to Pass extension

[marcocen]

This adds an additional action to copy OTP using the pass-otp extension.

The check for the extension isn't pretty but the otp extension doesn't add a binary to $PATH, just a file to /usr/local/lib/password-store/extensions (if installed with the source Makefile) o /usr/lib/password-store/extensions (as installed by some distributions).