Enable OTP support on mac

[wosc]

If I understand #327 and #394 correctly, this (very useful :) functionality is currently only enabled on Linux, and I'm not sure why? I can install and use pass-otp (with dependencies oathtool etc) just fine via homebrew (brew install pass-otp). Is there anything else missing on macOS? Or could you kindly just remove the "if not macOS" ifdefs for the next release? Thanks!