For subcommand names, replace underscores by dashes

[anntzer]

Options already implicitly replace underscores in their names by dashes in the CLI. For consistency, it would be appreciated if subcommands did the same. The options I see are

• break backcompatibilty and make this nonconfigurable.
• make this configurable (seems a bit overkill and inconsistent with the behavior for flags)
• hack into add_subparsers to map both ./foo.py bar_baz and ./foo.py bar-baz to the same function, preferably while displaying only one of them in the help string.

[evanunderscore]

For simplicity I'm fine with breaking backwards compatibility and bumping the major version.

[anntzer]

We may need to be careful with subcommand names that currently start with an underscore, though.