Allow setting left AND right edges

[RyanRamchandar]

For example a Point shape can have a pointed left and right edge be pointed.

Sample

Point(
  triangleHeight: 30.0,
  edge: Edge.LEFT_AND_RIGHT,
  child: Container(
    color: Colors.orange,
    width: 100.0,
    height: 100.0,
    child: Center(child: Text('80')),
  ),
),