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/readIndexConfig.js

@@ -0,0 +1,56 @@
+import path from 'path';
+import {
+    expect
+} from 'chai';
+import readIndexConfig from '../src/utilities/readIndexConfig';
+
+const fixtures = {
+  noIndex: path.resolve(__dirname, 'fixtures/read-index-config/no-index'),
+  withConfig: path.resolve(__dirname, 'fixtures/read-index-config/with-config'),
+  withInvalidConfig: path.resolve(__dirname, 'fixtures/read-index-config/with-invalid-config'),
+  withoutConfig: path.resolve(__dirname, 'fixtures/read-index-config/without-config')
+};
+
+const expectedValues = {
+  noIndex: {},
+  withConfig: {
+    ignore: ['/foo.js$/']
+  },
+  withoutConfig: {}
+};
+
+describe('readIndexConfig()', () => {
+  context('When valid config is defined', () => {
+    it('reads config object', () => {
+      const config = readIndexConfig(fixtures.withConfig);
+
+      expect(config).to.deep.equal(expectedValues.withConfig);
+    });
+  });
+
+  context('When invalid config is defined', () => {
+    it('should throw an error', () => {
+      const wrappedReadIndexConfig = () => {
+        readIndexConfig(fixtures.withInvalidConfig);
+      };
+
+      expect(wrappedReadIndexConfig).to.throw(/Configuration object must be a valid JSON./);
+    });
+  });
+
+  context('When config is NOT defined', () => {
+    it('returns an empty object', () => {
+      const config = readIndexConfig(fixtures.withoutConfig);
+
+      expect(config).to.deep.equal(expectedValues.withoutConfig);
+    });
+  });
+
+  context('When index file doesn\'t exist', () => {
+    it('returns an empty object', () => {
+      const config = readIndexConfig(fixtures.withoutConfig);
+
+      expect(config).to.deep.equal(expectedValues.noIndex);
+    });
+  });
+});