(test): add code coverage configuration

- ignore test-utils/ directory in coverage
- update gitignore with coverage/ directory
  - and fix missing newline

Author: agilgur5

.gitignore

@@ -1,9 +1,10 @@
 build/
+coverage/
 
 npm-debug.log*
 node_modules/
 
 *.tgz
 
 .DS_Store
-*~
+*~

jest.config.js

@@ -10,5 +10,9 @@ module.exports = {
   transform: {
     // use babel-jest@23 for babel@6 support (https://github.com/facebook/jest/issues/8230#issuecomment-479470547)
     '\\.js$': require.resolve('babel-jest')
-  }
+  },
+  coveragePathIgnorePatterns: [
+    '/node_modules/', // default
+    '<rootDir>/test-utils/' // ignore test-utils
+  ]
 }