Added some tests

types/d3-graphviz/d3-graphviz-tests.ts

@@ -0,0 +1,16 @@
+import { select, BaseType } from 'd3-selection';
+import * as d3Graphviz from 'd3-graphviz';
+
+let graphviz: d3Graphviz.Graphviz<BaseType, any, BaseType, any>;
+
+// Can get a graphviz render using extension method on d3-select
+graphviz = select('test').graphviz();
+
+// Can use graphviz method as selector
+graphviz = d3Graphviz.graphviz('test');
+
+// Can get the options
+const options: d3Graphviz.GraphvizOptions = d3Graphviz.graphviz('').options();
+
+// Can set the options
+graphviz = graphviz.options(options);