add build artifacts to gitignore

Author: tarekrached

.gitignore

@@ -2,3 +2,16 @@ node_modules
 npm-debug.log
 examples/bundle.js
 yarn.lock
+
+/PivotTable.js
+/PivotTable.js.map
+/PivotTableUI.js
+/PivotTableUI.js.map
+/PlotlyRenderers.js
+/PlotlyRenderers.js.map
+/TableRenderers.js
+/TableRenderers.js.map
+/Utilities.js
+/Utilities.js.map
+/__tests__/*
+/pivottable.css