startDrawingMode(mode, option) {
    return this._graphics.startDrawingMode(mode, option);
  }
Start a drawing mode. If the current mode is not 'NORMAL', 'stopDrawingMode()' will be called first.
@param {String} mode Can be one of <I>'CROPPER', 'FREE_DRAWING', 'LINE_DRAWING', 'TEXT', 'SHAPE'</I>
@param {Object} [option] parameters of drawing mode, it's available with 'FREE_DRAWING', 'LINE_DRAWING'
 @param {Number} [option.width] brush width
 @param {String} [option.color] brush color
 @param {Object} [option.arrowType] arrow decorate
   @param {string} [option.arrowType.tail] arrow decorate for tail. 'chevron' or 'triangle'
   @param {string} [option.arrowType.head] arrow decorate for head. 'chevron' or 'triangle'
@returns {boolean} true if success or false
@example
imageEditor.startDrawingMode('FREE_DRAWING', {
     width: 10,
     color: 'rgba(255,0,0,0.5)'
});
imageEditor.startDrawingMode('LINE_DRAWING', {
     width: 10,
     color: 'rgba(255,0,0,0.5)',
     arrowType: {
         tail: 'chevron' // triangle
     }
});