fix: cross platform open

package.json

@@ -21,9 +21,7 @@
     "pretest": "npm run build",
     "test": "npm run only:test",
     "precommit": "npm test",
-    "htmlcov": "npm run htmlcov:$(bash -c 'echo $OSTYPE')",
-    "htmlcov:linux-gnu": "xdg-open coverage/index.html",
-    "htmlcov:darwin": "open coverage/index.html",
+    "htmlcov": "open-cli coverage/index.html",
     "clean": "rm -rf coverage/ dist/ .nyc_output/"
   },
   "dependencies": {},
@@ -43,6 +41,7 @@
     "mocha": "6.1.4",
     "mocha-junit-reporter": "1.22.0",
     "nyc": "14.1.0",
+    "open-cli": "5.0.0",
     "source-map-support": "0.5.12",
     "typescript": "3.4.5"
   },