HPC should allow JSON output

Motivation

HPC is sort of neglected. I see it has an XML output option but no JSON output option. Most of the world operates on JSON or prefers to as the format is easier to read (as a human), better libraries exist, and sufficient.

Worse, the output format was shortsightedly made as a single flag of --xml-output instead of a more extensible --output-format <format>.

Proposal

We should deprecated the flag, introduce a ne w--output-format flag and add JSON support. Aeson or similar would make it easy, but I'm not sure where the community stands on adding build dependencies to HPC - are we conservative as with Cabal or more lenient?