Conversion to click

This includes a mess of changes:
 * stricter option parsing (ex: global options are specified directly after sl)
 * no prefix-based option matching that doc-opt did
 * entrypoint-based loading
 * Several CLI cleanups:
 ** Remove help command
 ** Renames dns subcommands to include zone- and record-
 ** Renames --hourly and --billing for `sl vs create` and `sl hardware create`
    to --billing=hourly or --billing=monthly.
 ** Many other subtle changes