Add gopass prompt

[patrickpichler]

The gopass prompt works basically the same as the pass prompt, but
it is calling the gopass specific command to retreive one otp token.

[patrickpichler]

I hope this is fine :)

Gopass works a bit different to pass, hence its own prompt provider :)

[mtibben]

@patrickpichler what are your thoughts on this #1006 (comment)

[patrickpichler]

I would favor a script prompt!

I had a quick look at how one could implement it, but given that I wanted a quick way
of it working, I decided to go with the less work intensive way of just providing another
prompt 🙂

[mtibben]

Closed in favour of #1123