Add .example(cmd, desc) feature

Add a `.example(cmd, desc)` feature.

Calling .example(cmd, desc) adds an "Examples:" section to the help
output.

### Calling

    .example('$0 something -f blah', 'performs magic')
    .example('$0 else -f blah', 'more magic')

### Results In

    Examples:
      ./your-script.js something -f blah    performs magic
      ./your-script.js else -f blah         more magic