Add codecov and badges (#94)

.travis.yml

@@ -1,4 +1,8 @@
 language: node_js
 node_js:
    - "lts/*"
+before_install:
+  - npm install -g codecov
 script: npm test
+after_success:
+  - codecov

README.md

@@ -1,5 +1,16 @@
 # post-robot [:]-\\-<
 
+[![build status][build-badge]][build]
+[![code coverage][coverage-badge]][coverage]
+[![npm version][version-badge]][package]
+
+[build-badge]: https://img.shields.io/github/workflow/status/krakenjs/post-robot/build?logo=github&style=flat-square
+[build]: https://github.com/krakenjs/post-robot/actions?query=workflow%3Abuild
+[coverage-badge]: https://img.shields.io/codecov/c/github/krakenjs/post-robot.svg?style=flat-square
+[coverage]: https://codecov.io/github/krakenjs/post-robot/
+[version-badge]: https://img.shields.io/npm/v/post-robot.svg?style=flat-square
+[package]: https://www.npmjs.com/package/post-robot
+
 Cross domain post-messaging on the client side, using a simple listener/client pattern.
 
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