Move to `braid help COMMAND` and `braid COMMAND --help`

[mattmccutchen]

Braid should accept the syntaxes braid help COMMAND and braid COMMAND --help, which are common in other commands that have subcommands, including git itself. In the current, idiosyncratic braid COMMAND help syntax, help looks like it is a single argument to the subcommand. We should deprecate that syntax (printing a warning along with the help text) and perhaps remove it after a while, allowing us to (e.g.) have a mirror named help.

Another thing for my long to-do list...