add custom autotype, open URLs, generate OTPs

This is a relatively big release. The following features have been
added:

- generate, copy, and autotype releases with the help of pass-otp
- URLs can be either opened in the default web browser or copied
- the ability to customize auto-type operation
- the ability to choose custom keys for username, url, and autotype
- a man page

The code has been through lots of refactoring in this release to make it
more modular and a few bugs were fixed as well.
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