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

src/bin/create-index.js

@@ -15,10 +15,17 @@ const argv = yargs
       type: 'boolean'
     }
   })
+  .options({
+    'banner': {
+      description: 'Add a custom banner at the top of the index file',
+      type: 'string'
+    }
+  })
   .example('create-index ./src ./src/utilities', 'Creates or updates an existing create-index index file in the target (./src, ./src/utilities) directories.')
   .example('create-index --update ./src ./tests', 'Finds all create-index index files in the target directories and descending directories. Updates found index files.')
   .argv;
 
 writeIndexCli(argv._, {
-  updateIndex: argv.update
+  updateIndex: argv.update,
+  banner: argv.banner
 });

src/utilities/createIndexCode.js

@@ -22,10 +22,22 @@ const buildExportBlock = (files) => {
   return importBlock;
 };
 
-export default (filePaths) => {
+export default (filePaths, options = {}) => {
   let code;
 
-  code = '// @create-index\n\n';
+  code = '';
+
+  if (options.banner) {
+    const banners = _.isArray(options.banner) ? options.banner : [options.banner];
+
+    banners.forEach((banner) => {
+      code += banner + '\n';
+    });
+
+    code += '\n';
+  }
+
+  code += '// @create-index\n\n';
 
   if (filePaths.length) {
     const sortedFilePaths = filePaths.sort();

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');