ci: add code coverage report

.travis.yml

@@ -21,3 +21,5 @@ after_script:
       snyk protect;
       snyk monitor;
     fi
+  - npm install codecov
+  - codecov

README.md

@@ -2,6 +2,7 @@
 
 [![npm version](https://badge.fury.io/js/hexo-yam.svg)](https://www.npmjs.com/package/hexo-yam)
 [![Build Status](https://travis-ci.com/curbengh/hexo-yam.svg?branch=master)](https://travis-ci.com/curbengh/hexo-yam)
+[![codecov](https://codecov.io/gh/curbengh/hexo-yam/branch/master/graph/badge.svg)](https://codecov.io/gh/curbengh/hexo-yam)
 [![NPM Dependencies](https://david-dm.org/curbengh/hexo-yam.svg)](https://david-dm.org/curbengh/hexo-yam)
 [![Known Vulnerabilities](https://snyk.io/test/npm/hexo-yam/badge.svg)](https://snyk.io/test/npm/hexo-yam)
 [![Greenkeeper badge](https://badges.greenkeeper.io/curbengh/hexo-yam.svg)](https://greenkeeper.io/)

package.json

@@ -49,7 +49,7 @@
   "jest": {
     "clearMocks": true,
     "collectCoverage": true,
-    "coverageDirectory": "coverage",
+    "coverageDirectory": "./coverage/",
     "testEnvironment": "node"
   },
   "greenkeeper": {