add custom heading component

[dehbmarques]

Related to #13.

I added one customHeadingComponent prop in the ColumnDefinition.
When a component is defined in this customHeadingComponent prop, it will be rendered instead of the default title + sortIcon.
eg:

class Header extends Component {

  render() {
    return (
      <span>Custom {this.props.title} {this.props.sortIcon}</span>
    );
  }
}

export default class App extends Component {

  render() {

    return (
      <Griddle data={fakeData}>
        <RowDefinition>
          <ColumnDefinition id="name" customHeadingComponent={Header} />
          <ColumnDefinition id="city" />
        </RowDefinition>
      </Griddle>
    );
  }
}

I kept the customHeadingComponent propType as an object, like the customComponent.
But shouldn't it be an element?

Great component btw. I've been using a lot.