chore(typings): add interface Config['cors'] (#12)

* chore(typings): add interface Config['cors']

usage:

```ts
// {app_root}/config/config.default.ts

import { Config as CorsConfig } from 'egg-cors'

export default (appInfo: EggAppConfig) => {
  const config = <PowerPartial<EggAppConfig> & CorsConfig> {}
  // ...
  config.cors = {
    origin: '*',
    allowMethods: 'GET,HEAD,PUT,POST,DELETE,PATCH,OPTIONS',
  }

  return config
}
```

index.d.ts

@@ -0,0 +1,9 @@
+
+declare module 'egg' {
+  export interface EggAppConfig {
+    cors: {
+      origin: string | (() => string);
+      allowMethods: string | string[];
+    }
+  };
+}

package.json

@@ -8,7 +8,8 @@
   "files": [
     "app",
     "config",
-    "app.js"
+    "app.js",
+    "index.d.ts"
   ],
   "repository": {
     "type": "git",