Output deprecation message when `sanitize` is called.

cc @chancancode
"This sanitize implementation" only makes sense when you are reading the code here, not so much when people simply upgraded their app and restarted the server (which is really who we are targeting).

Perhaps something like...

```
In Rails 5, the underlying implementation of the `sanitize` helper will be replaced by a __faster__,
__stronger__, __better__ implementation :wink: (no, not really, please rewrite this line!). You can
opt into the new implementation today (and remove this warning) by adding the `rails-html-sanitizer`
gem to your Gemfile. Refer to the ... for the differences between the two implementations.
```

I'll leave it to you to digest and improve this :grin: 

But don't use heredoc! https://github.com/rails/rails/commit/8c52480ba6071a12b97f124d9f21df2c39df3ed3