Multiple level heading

[alemangui]

Hello and thanks for this great library.

I'd like to make use of different heading levels than the current default of h1.

One short-term option could be to be able to configure the heading level used by Trix in django-prose, as described in this Trix issue : basecamp/trix#972, where they mention a conf key like :

Trix.config.blockAttributes.heading1.tagName = 'h2';

Is there a way to set it in django-prose ?

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Ideally though, we could choose the heading level for SEO and accessibility purposes. This PR contains an interesting layout that I think could be very useful : basecamp/trix#861

[alemangui]

For info we created this PR in our project which allowed us to add the heading levels. If you think a similar approach could be baked into the library let me know, maybe I could help develop it.