Support running tests with code coverage collection

Coverage will only be measured if the `MEASURE_COVERAGE=true`
environment variable is set. We'll set that on CI, but it won't be
required to collect code coverage every single time we run the tests
locally.

package.json

@@ -8,7 +8,7 @@
   },
   "scripts": {
     "start": "node lib/index.js",
-    "test": "mocha"
+    "test": "if [ \"${MEASURE_COVERAGE}\" = 'true' ]; then nyc mocha; else mocha; fi"
   },
   "repository": {
     "type": "git",