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/writeIndex.js

@@ -8,29 +8,51 @@ import {
 import writeIndex from '../src/utilities/writeIndex';
 import codeExample from './codeExample';
 
+const readFile = (filePath) => {
+  return fs.readFileSync(filePath, 'utf8');
+};
+
+const removeFile = (filePath) => {
+  fs.unlinkSync(filePath);
+};
+
+const appendToFile = (filePath, content) => {
+  fs.appendFileSync(filePath, content, 'utf-8');
+};
+
 const fixturesPath = path.resolve(__dirname, 'fixtures/write-index');
 
 describe('writeIndex()', () => {
   it('creates index in target directory', () => {
     const indexFilePath = path.resolve(fixturesPath, 'mixed/index.js');
 
-    try {
-      fs.unlinkSync(indexFilePath);
-
-      // eslint-disable-next-line no-empty
-    } catch (error) {
-
-    }
-
+    removeFile(indexFilePath);
     writeIndex([path.resolve(fixturesPath, 'mixed')]);
-
-    const indexCode = fs.readFileSync(indexFilePath, 'utf8');
+    const indexCode = readFile(indexFilePath);
 
     expect(indexCode).to.equal(codeExample(`
 // @create-index
 
 export { default as bar } from './bar';
 export { default as foo } from './foo.js';
-        `));
+    `));
+  });
+
+  it('creates index with config in target directory', () => {
+    const indexFilePath = path.resolve(fixturesPath, 'with-config/index.js');
+    // eslint-disable-next-line
+    const ignoredExportLine = `export { default as bar } from './bar.js';`;
+
+    appendToFile(indexFilePath, ignoredExportLine);
+    expect(readFile(indexFilePath).includes(ignoredExportLine)).to.equal(true);
+
+    writeIndex([path.resolve(fixturesPath, 'with-config')]);
+    const indexCode = readFile(indexFilePath);
+
+    expect(indexCode).to.equal(codeExample(`
+// @create-index {"ignore":["/bar.js$/"]}
+
+export { default as foo } from './foo.js';
+    `));
   });
 });