Allow blocking elements by selector (#50)

* Extract method (isElementBlocked) and add tests

* Add blockSelector argument to snapshot

If blockSelector is passed, it will be matched against the element.

Reasoning: Mutating class names can get messy, so providing another hook
helps keep code clean by using data-attributes instead.

package.json

@@ -34,12 +34,14 @@
   "homepage": "https://github.com/rrweb-io/rrweb-snapshot#readme",
   "devDependencies": {
     "@types/chai": "^4.1.4",
+    "@types/jsdom": "^16.2.4",
     "@types/mocha": "^5.2.5",
     "@types/node": "^10.11.3",
     "@types/puppeteer": "^1.12.4",
     "chai": "^4.1.2",
     "cross-env": "^5.2.0",
     "jest-snapshot": "^23.6.0",
+    "jsdom": "^16.4.0",
     "mocha": "^5.2.0",
     "puppeteer": "^1.15.0",
     "rollup": "^0.66.4",