feat: ability to add banner (#20)

Lets you add a custom banner.

```sh
create-index src -u --banner "/* eslint-disable */"
```
```js
/* eslint-disable */

// @create-index

import xxx ...
```

src/utilities/writeIndexCli.js

@@ -35,7 +35,9 @@ export default (directoryPaths, options = {}) => {
 
     const siblings = readDirectory(directoryPath);
 
-    const indexCode = createIndexCode(siblings);
+    const indexCode = createIndexCode(siblings, {
+      banner: options.banner
+    });
 
     const indexFilePath = path.resolve(directoryPath, 'index.js');