hplip update and fixes

This pull request does several things:
* Bumps hplip version from 3.11.1 to 3.14.4, which brings support for many new devices.
* Adds support for HP's proprietary binary plugin, required to print with many new devices (especially LaserJet models). With the proprietary plugin enabled, the package is unfree, so the plugin is disabled by default. Besides the default `hplip`, a second top-level attribute `hplipWithPlugin` is provided that has the plugin enabled.
* Begins to fix scanning by adding missing dependencies. Scanning still doesn't work because the SANE dynamic loader can't find the driver, but fixing that will require completely restructuring how we do SANE backends.
* Given that the package is so badly out of date and no maintainer is listed, adds myself as maintainer.