feat: add ScatterTracer (#14)

Author: Jeff-Tian

Diff for: src/ScatterTracer.ts

@@ -0,0 +1,4 @@
+import { Array2DTracer } from './';
+
+export default class ScatterTracer extends Array2DTracer {
+}

Diff for: src/index.ts

@@ -9,3 +9,4 @@ export { default as Array2DTracer } from './Array2DTracer';
 export { default as Array1DTracer } from './Array1DTracer';
 export { default as ChartTracer } from './ChartTracer';
 export { default as GraphTracer } from './GraphTracer';
+export { default as ScatterTracer } from './ScatterTracer';