feat: add ignore option (fix #31) (#37)

- Add functionality to ignore files based on provided
regular expressions
- Support writeIndex and writeIndexCli
- in index.js config must be specified as below

    // @create-index {"ignore":["/foo.js"]}

src/utilities/hasIndex.js

@@ -0,0 +1,14 @@
+import fs from 'fs';
+import path from 'path';
+
+export default (directoryPath) => {
+  const indexPath = path.resolve(directoryPath, 'index.js');
+
+  try {
+    fs.statSync(indexPath);
+
+    return true;
+  } catch (error) {
+    return false;
+  }
+};