[Tests] add npm run coverage

Author: ljharb

.gitignore

@@ -5,3 +5,7 @@ node_modules
 npm-shrinkwrap.json
 package-lock.json
 yarn.lock
+
+# Coverage
+coverage
+.nyc_output

.nycrc

@@ -0,0 +1,17 @@
+{
+  "all": true,
+  "check-coverage": true,
+  "instrumentation": false,
+  "sourceMap": false,
+  "reporter": "html",
+  "lines": 96,
+  "statements": 95,
+  "functions": 96,
+  "branches": 92,
+  "exclude": [
+    "coverage",
+    "example",
+    "test",
+    "test-core-js.js"
+  ]
+}

.travis.yml

@@ -28,6 +28,8 @@ env:
 matrix:
   fast_finish: true
   include:
+    - node_js: "node"
+      env: COVERAGE=true
     - node_js: "8.5"
       env: TEST=true ALLOW_FAILURE=true
     - node_js: "8.4"

package.json

@@ -5,12 +5,14 @@
   "main": "index.js",
   "devDependencies": {
     "core-js": "^2.5.1",
+    "nyc": "^10.3.2",
     "tape": "^4.8.0"
   },
   "scripts": {
     "test": "npm run tests-only",
     "pretests-only": "node test-core-js",
-    "tests-only": "tape test/*.js"
+    "tests-only": "tape test/*.js",
+    "coverage": "nyc npm run tests-only"
   },
   "testling": {
     "files": [