chore: setup code-coverage (#53)

Fixes: #31

Author: ofrobots

.gitignore

@@ -1,8 +1,9 @@
 .DS_Store
+.nyc_output
 .vscode
 build
+coverage
 node_modules
 npm-debug.log
 yarn-error.log
 yarn.lock
-staging

.travis.yml

@@ -6,3 +6,7 @@ node_js:
   - "4"
   - "6"
   - "8"
+
+script:
+  - npm run test
+  - npm run codecov