Various arg_botcmd improvements

[zoni]

This PR introduces a number of enhancements to how the arg_botcmd decorator works:

• Ensure arg_botcmd error and help output goes into chat and avoid raising SystemExit

By default, argparse will print errors, usage and help text directly to
stdout/stderr. It will also raise SystemExit to make the program stop
with an appropriate error code.

While this is appropriate for a CLI program, it is horrible for Err. The
SystemExit causes some strange behaviour where it doesn't make the bot
exit, but does prevent some processing (!restart no longer works, for
example) and the printing to stdout/stderr means you get no feedback
when issuing commands with incorrect arguments.

With these changes, all this output is nicely returned to the user,
instead.

• Ensure the command name is used in the usage as well, instead of the
  default sys.arv[0] which resulted in one of err.py, errbot or
  run_tests.py.

• Convert — to -- when parsing arg_botcmds

  Some clients (looking at you, Telegram) convert certain ASCII character
  combinations to nicer-looking unicode representations, which break the
  argument parser. Account for this and undo the conversion to avoid user
  frustration.

• Make "argument unpacking" on arg_botcmd optional

With this change it becomes possible to define a single args argument
to hold all the argparser arguments, removing the need to define them in
the function signature itself.

This is useful especially on commands with lots of argument flags, as
otherwise you have to make sure you define all your arguments in just
the right order, which is very error prone.

[zoni]

Preview of the usage/help text on Telegram (which removes blank lines for some reason):