command: prevent fall back to global usage for subcommand option errors

[HoyeonRhee]

Argparse reports unknown arguments from top-level parser even after
selecting a subcommand, so command like "b4 dig --unknown-arg" prints
the global usage instead of the dig usage.

To handle, this commit set a subparser attribute for each subcommand
parser, then use parse_known_args() in cmd() to report leftover arguments
from the selected subparser instead of the top-level parser.

$ b4 dig --unknown-arg

# before:
usage: b4 [-h] [--version] [-d] [-q] [-n] [--offline-mode] [--no-stdin]
          [-c NAME=VALUE]
          {mbox,am,shazam,review,pr,ty,diff,kr,prep,trailers,send,dig} ...
b4: error: unrecognized arguments: --unknown-arg

# after:
usage: b4 dig [-h] [-c COMMITISH] [-C] [-a | -m DEST | -w]
b4 dig: error: unrecognized arguments: --unknown-arg

Fixes: #75