Add support for ctx.roundRect

https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/roundRect

src/Context.ts

@@ -58,6 +58,7 @@ var COMMA = ',',
     'putImageData',
     'quadraticCurveTo',
     'rect',
+    'roundRect',
     'restore',
     'rotate',
     'save',
@@ -586,6 +587,18 @@ export class Context {
   rect(x: number, y: number, width: number, height: number) {
     this._context.rect(x, y, width, height);
   }
+  /**
+   * roundRect function.
+   * @method
+   * @name Konva.Context#roundRect
+   */
+  roundRect(x: number, y: number, width: number, height: number, radii: number) {
+    if (this._context.roundRect) {
+      this._context.roundRect(x, y, width, height, radii);
+    } else {
+      this._context.rect(x, y, width, height);
+    }
+  }
   /**
    * putImageData function.
    * @method

test/unit/Context-test.ts

@@ -22,6 +22,7 @@ describe('Context', function () {
     'arc',
     'arcTo',
     'rect',
+    'roundRect',
     'ellipse',
     'fill',
     'stroke',