Adding -B option as an alias to --skip-bundle

On creating new project, when we don't want middleman to execute `bundle install`, we use `--skip-bundle` option.

    $ middleman init my_new_project --skip-bundle

That's happy enough, but in rails, `--skip-bundle` option has an alias, `-B` short option, so maybe middleman can have the same interface.

    $ middleman init my_new_project -B

How do you feel about it?