(ci/test): add CodeCov test coverage reporting

- change CI to output and upload coverage reports

- (docs): add coverage badge to README

Author: agilgur5

.travis.yml

@@ -3,5 +3,8 @@ language: node_js
 node_js: '8.9'
 
 before_script: npm run lint
-script: npm test
-after_script: npm run test:pub
+script: npm test -- --coverage
+after_script:
+  # upload coverage reports to CodeCov
+  - bash <(curl -s https://codecov.io/bash)
+  - npm run test:pub

README.md

@@ -11,6 +11,7 @@
 [![dw](https://img.shields.io/npm/dw/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 <br><!-- status / activity -->
 [![build status](https://img.shields.io/travis/agilgur5/react-signature-canvas/master.svg)](https://travis-ci.org/agilgur5/react-signature-canvas)
+[![code coverage](https://img.shields.io/codecov/c/gh/agilgur5/react-signature-canvas/master.svg)](https://codecov.io/gh/agilgur5/react-signature-canvas)
 <br>
 [![NPM](https://nodei.co/npm/react-signature-canvas.png?downloads=true&downloadRank=true&stars=true)](https://npmjs.org/package/react-signature-canvas)