Add type definition (#10)

* Add type definition

* Make file prop optional

Author: TaylorBeeston

package.json

@@ -4,7 +4,9 @@
   "description": "Github Gist React component",
   "main": "lib/index.js",
   "module": "es/index.js",
+  "types": "types",
   "files": [
+    "types/index.d.ts",
     "lib",
     "es"
   ],

types/index.d.ts

@@ -0,0 +1,10 @@
+import { PureComponent } from "react";
+
+export interface GistProps {
+  id: string;
+  file?: string;
+}
+
+declare class Gist extends PureComponent<GistProps, any> {}
+
+export default Gist;