Multiple level heading

The interface of the editor is cool, but there is restricting feature in it. Such as not been able to select a specific heading as a user. (from h1 to h6).

We can select only 1 heading-level, default `h1`.

Something like: https://github.com/basecamp/trix/pull/861 would be great !

If there is multiple heading level by default or a way to add custom tags / buttons in the menu without rebuilding the whole library or template, that would be awesome.

##### Details

* Trix version: v1 and v2

The issue names no files, tests, or entry points; start by locating the editor's heading-level selection and toolbar customization behavior, then compare the requested direction with basecamp/trix#861. Done means supporting multiple heading levels or a documented way to add custom tags or buttons without rebuilding the library or template.

Written by the indexing model from the issue text.