hplip: update and fix scanning

This pull request addresses several issues with the hplip package:
* The package is very out of date. This PR bumps from version 3.11.1 to 3.14.4. The new version adds support for the many devices HP has introduced in the last 3(!) years.
* Because the package is so out of date and no maintainer is listed, I have added myself as maintainer.
* Some new HP devices (particularly, LaserJet all-in-one models) require a proprietary binary plugin. The plugin is disabled by default because it is unfree, but a top-level attribute, `hplipWithPlugin` is added which has the plugin.
* The SANE backend for HP scanners is now installed. I have changed the NixOS module for SANE so that the hpaio backend can be selected at runtime. There was a problem before where SANE could only find backends in the sane-backend package. I actually extended the NixOS module so that it would be trivial to add *any* backend from another package (what SANE calls "external backends").