Plug-ins won't do. It needs to be built into the browser itself, turned on by default, otherwise the vast majority of users will never have the functionality.
With how most browsers are engineered, I don't think there's a need for putting it into the workings of a browser. It's a perfect use-case for browser extensions or releasing something similar to Firefox ESR with a bundle of pre-installed and pre-configured extensions (it's essentially the idea behind the TOR Browser, at least its early releases). Plus consider all the outcry resulting from making it opt-out. Libel/slander, "who watches the watchers", accusations of censorship, genuine false-positives, media outlets using the tool against competitors, and who knows what kind of legal implications. Those always follow even in much more benign changes when it comes to content flagging.