Render blocks of documentation files according to their version

[octopuss]

It will be nice to have possibility to render parts of documents according to their version. Currently we can set since only for the whole article or a section, see #7.

Proposed format :

--- 2.0

 this content belongs, and is rendered only if documentation is in version 2.0 or higher

---

[borekb]

--- is a Markdown syntax for <hr>, I think I'd use something similar to the [TODO ...] format that we already have. Perhaps something like

Some paragraph here

[since: 3.0]
This paragraph applies to 3.0 only.

Some other standard paragraph here...

The tag would apply to the closest block element so we could also filter out things like tables, lists etc.

But it's not quite important at the moment, let's leave this for the future when we need this feature.

[JanVoracek]

@borekb: And perhaps some optional end mark like [/] for multiple paragraphs / paragraphs with list etc.?

[borekb]

Perhaps :) Let's leave it till we need it.