Add custom html classes to gutenberg wrapper

• Block Filters are used to customize the class that wordpress already adds using following format wp-block-[plugin_name]-[block-name]. You can access that using this.props.className.
• You can also add the additional classes using Advanced Panel which is present on right sidebar.
• In the end you can use React.js component state to define custom classes (pseudo code is given below).

(Create separate file for this and pass into edit method inside registerBlockType for better structuring else you can write this whole code)

class Testing extends Component {
  constructor() {
    this.state = {
      customClasses: null
    }
  }

  render() {
    <InspectorControls>
      <TextControl
        label="Custom CSS Classes"
        value={ this.state.customClasses }
        onChange={ ( content ) => setState( { customClasses: content } ) }
      />
    </InspectorControls>
  }
}