Rendering markdown in the terminal

[klntsky]

As for now we output raw markdown, but this should be changed.

We need to wrap one of Node markdown renderers into a PS interface.

Candidates are:

https://github.com/trabus/markdown-it-terminal

https://github.com/mikaelbr/marked-terminal

The first is preferred, since we are already using markdown-it for HTML rendering.

Contributors are very welcome!