[New Feature] Add border radius customization for rects and header boxes

[weebao]

closes #94

interface Rect {
  x: number;
  y: number;
  width: number;
  height: number;
  round: number; // NEW
}

interface Header {
  label?: string;
  property?: string;
  width?: number;
  align?: string; //default 'left'
  valign?: string;
  headerColor?: string; //default '#BEBEBE'
  headerOpacity?: number; //default '0.5'
  headerRound?: number; // NEW - default '0'
  headerAlign?: string; //default 'left'
  columnColor?: string;
  columnOpacity?: number;
  renderer?: (
    value: any,
    indexColumn?: number,
    indexRow?: number,
    row?: number,
    rectRow?: Rect,
    rectCell?: Rect
  ) => string;
}