(ci/test): add CodeCov test coverage reporting

- change CI to output and upload coverage reports

- (docs): add coverage badge to README

.travis.yml

@@ -2,5 +2,8 @@ language: node_js
 # default is the very old 0.10.48; match local version instead
 node_js: '8.9'
 
-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

@@ -12,6 +12,7 @@
 <br><!-- status / activity -->
 [![typings](https://img.shields.io/npm/types/mst-persist.svg)](https://github.com/agilgur5/mst-persist/blob/master/src/index.ts)
 [![build status](https://img.shields.io/travis/agilgur5/mst-persist/master.svg)](https://travis-ci.org/agilgur5/mst-persist)
+[![code coverage](https://img.shields.io/codecov/c/gh/agilgur5/mst-persist/master.svg)](https://codecov.io/gh/agilgur5/mst-persist)
 <br>
 [![NPM](https://nodei.co/npm/mst-persist.png?downloads=true&downloadRank=true&stars=true)](https://npmjs.org/package/mst-persist)