Parse Markdown into document nodes

# Problem to solve

Currently we have the ability to render a rich text, where the text is represented as a tree of `Node`s. However, that's not very convenient from user's perspective. A better approach would be to allow some common textual representation -- such as Markdown -- to be translatable into a node tree so that it can later be rendered.

# Proposal

A special utility class to translate a markdown snippet into a node tree.