feat: support executable root command

[mdonnalley]

Allow CLIs to have an executable root command (instead of just showing help output). Behavior is opt-in so existing CLIs should be unaffected

TODO: Submit PR to update documentation for command discovery strategies once this is merged

Before:

$ my-cli => shows help output

After

$ my-cli => executes command logic

Usage

In the oclif section of your package.json

    "commands": {
      "strategy": "pattern",
      "includeRoot": true,
      "target": "./lib/commands"
    },

The includeRoot property tells oclif to use the Command class found in lib/commands/index.js