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 ...
```

README.md

@@ -86,6 +86,7 @@ Options:
                 files that start with "'create index';\n" (create-index index
                 file). Updates found index files. Does not create new index
                 files.                                [boolean] [default: false]
+  --banner      Add a custom banner at the top of the index file        [string]
 
 Examples:
   create-index ./src ./src/utilities   Creates or updates an existing