fix(DateTimeControl): add control to barrel export file (#104)

Author: ashmortar

src/index.ts

@@ -15,5 +15,6 @@ export { ObjectControl } from "./controls/ObjectControl"
 export { ObjectArrayControl } from "./controls/ObjectArrayControl"
 export { PrimitiveArrayControl } from "./controls/PrimitiveArrayControl"
 export { TextControl } from "./controls/TextControl"
+export { DateTimeControl } from "./controls/DateTimeControl"
 export { UnknownControl } from "./controls/UnknownControl"
 export { VerticalLayout } from "./layouts/VerticalLayout"