allow custom shapes in Perimeter Anchors

[sporritt]

it would be nice to support custom, named, shapes in the perimeter anchors. one user added this to the source code, for example:

"Parallelogram":function() {
return _shape([
[ 0.25, 0, 1, 0 ], [ 1, 0, 0.75, 1 ], [ 0.75, 1, 0, 1 ], [ 0, 1, 0.25, 0 ]
]);
},

so instead if we could say something like

jsPlumb.Anchors.Perimeter.Parallelogram = [ 0.25, 0, 1, 0 ], [ 1, 0, 0.75, 1 ], [ 0.75, 1, 0, 1 ], [ 0, 1, 0.25, 0 ];

it would be useful for people.