Fixed commenting on Facebook

Fixed a regression in f87e7d0f71b9373dfa71384085370923317d80db that causes comments to be mangled as they are added on Facebook. Use deleteData() / insertData() instead of replaceData(). For some reason Facebook fails to react to the latter through its mutation observers. This change shouldn’t affect performance much because it’s still limited to just the one word.

Fixes #105.