- command reference now includes usage for all Spack commands as output
by `spack help`. Each command usage links to any related section in
the docs.
- added `spack commands` command which can list command names,
subcommands, and generate RST docs for commands.
- added `llnl.util.argparsewriter`, which analyzes an argparse parser and
calls hooks for description, usage, options, and subcommands
