Support Markdown-formatted text inside HTML blocks

Previously `<div>` elements couldn’t wrap Markdown-formatted content inside. This is because `<div>` is a block element and plain Markdown does not format the content of such.

With this change the behavior for HTML block elements is configurable now. To enable Markdown processing add a "markdown" attribute with a value of `1` or `span` to the element. An empty SGML attribute, as in `<div markdown>` also enables Markdown processing. To disable pass a value of `0` or `block`.

This feature was introduced by PHP Markdown Extra.

Limitations:

* In contrast to PHP Markdown Extra this doesn't permit to disable Markdown processing for inline elements yet.
* The value of this attribute is not inherited by child elements yet.