Allow numbering custom elements as figure/table/block

[Jellby]

Add the option to have other elements (added by an extension) numbered as figure, table or code block, by specifying the attribute "figtype".

[tk0miya]

Now, we introduce new API Sphinx.add_enumerated_node to support this.
More details, please refer #2317.

Anyway, your pull request has become of the wake of the feature.
Thank you for great proposal.

[eudoxos]

Coming back here, I see add_enumerable_node was merged in 1.4; how is it supposed to be used? It is not documented at all from the user perspective.

[paternal]

Coming back here, I see add_enumerable_node was merged in 1.4; how is it supposed to be used? It is not documented at all from the user perspective.

@eudoxos: I managed to use it in sphinxcontrib-proof. You can see it in action there.

Creating a dummy extension to show how to use add_enumerable_node() (and documenting it) is on my TODO list, but with a low priority. So if you, random passerby, want to do it, go ahead!

[paternal]

I wrote such a dummy extension. I offered to the sphinx team to adopt it as a part of @sphinx-contrib (see #5567).