Whitelist/Blacklist Specific Tags

**Is your feature request related to a problem? Please describe.**
Our specific use case is related to the use of H1 tags.

We are using the editor to produce support documentation and blog posts with a strict HTML output, so editors won't have too much influence to the actual design or layout of the generated page. For SEO purposes, it's important to only use one H1 tag, and make sure it's equal/similar to the `` element, meta tags and so on.

So for that reason, giving the user the option to insert multiple H1's wherever they like, is not a great way to ensure proper output.

**Describe the solution you'd like**
We would like the option to either create a strict whitelist or blacklist of tags that either will or will not be processed by the editor.

**Describe alternatives you've considered**
We are already stripping blacklisted tags from the generated HTML on the server side and in the server-side generated preview. But this does not have any effect on what the user sees while editing.