Add support for <markdown> and <code-block> tags

[philidem]

For example:

<code-block lang="js">
function abc(x, y, z) {
    // do something
}
</code-block>

<markdown>
# Overview
This is the overview
## This is a section
Blah blah blah.
</markdown>

[patrick-steele-idem]

No longer applicable. Closing PR