add description field from __docgenInfo for prop table for info plugin

[konsumer]

Issue:

I'd like to see description from __docgenInfo in the props documentation in info plugin

What I did

Added code to PropTable

How to test

Make a story with a component that has documented (via docgen-style text) props, and the description will show up.

Like this:

/** cool button-type thing */
const Button = ({children, onClick}) => (
  <button onClick={onClick}>{children}</button>
)
Button.propTypes = {
  /** Things and stuffs */
  onClick: PropTypes.func
}

storiesOf('Button', module)
  .addWithInfo('demo - David', 'Button with children set to "David"', () => (
    <Button onClick={action('clicked')}>David</Button>
  ))

You will see a description field in the table that says Things and stuffs.

[ndelangen]

Thank you @konsumer !