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"]}

test/fixtures/read-index-config/with-config/index.js

@@ -0,0 +1 @@
+// @create-index {"ignore": ["/foo.js$/"]}

test/fixtures/read-index-config/with-invalid-config/index.js

@@ -0,0 +1 @@
+// @create-index {ignore: 'foo'}

test/fixtures/read-index-config/without-config/index.js

@@ -0,0 +1 @@
+// @create-index

test/fixtures/write-index/with-config/index.js

@@ -0,0 +1,4 @@
+// @create-index {"ignore":["/bar.js$/"]}
+
+export { default as foo } from './foo.js';
+