Add typings

Add typescript typings for the main function.

Author: Tom

index.d.ts

@@ -0,0 +1,27 @@
+export default function(
+  text: string,
+  shade: 50 | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900,
+): {
+  backgroundColor: string;
+  color: string;
+  materialColorName:
+    | "Red"
+    | "Pink"
+    | "Purple"
+    | "Deep Purple"
+    | "Indigo"
+    | "Blue"
+    | "Light Blue"
+    | "Cyan"
+    | "Teal"
+    | "Green"
+    | "Light Green"
+    | "Lime"
+    | "Yellow"
+    | "Amber"
+    | "Orange"
+    | "Deep Orange"
+    | "Brown"
+    | "Grey"
+    | "Blue Grey";
+};