Add code coverage

.gitignore

@@ -1,2 +1,3 @@
 node_modules
 dist
+.nyc_output

.travis.yml

@@ -1,3 +1,4 @@
 language: node_js
 node_js: node
 script: npm run lint && npm test
+after_success: npm run coverage

package.json

@@ -11,16 +11,19 @@
     "babel-plugin-add-module-exports": "^0.1.2",
     "babel-preset-es2015": "^6.6.0",
     "chai": "^3.5.0",
+    "coveralls": "^2.11.9",
     "eslint": "^2.8.0",
     "mocha": "^2.4.5",
+    "nyc": "^6.4.2",
     "pre-commit": "^1.1.2"
   },
   "scripts": {
     "prebuild": "rm -rf dist",
     "build": "babel -d dist src",
     "pretest": "npm run build",
-    "test": "mocha test",
+    "test": "nyc mocha test",
     "lint": "eslint src",
+    "coverage": "nyc report --reporter=text-lcov | coveralls",
     "prepublish": "npm run build"
   },
   "pre-commit": [