test: added script to get coverage report

PR-URL: #52
Credit: @mikemimik
Close: #52
Reviewed-by: @isaacs

Author: Michael Perrotte

package.json

@@ -24,6 +24,7 @@
     "postrelease": "npm publish && git push --follow-tags",
     "pretest": "standard",
     "release": "standard-version -s",
+    "test:coverage": "tap --coverage-report=html -J --100 --no-esm test/*.js",
     "test": "tap -J --100 --no-esm test/*.js"
   },
   "dependencies": {