Add support for images and triple backticks code block for Markdown

[Kapranov98]

It would be nice to add an image display support for Markdown Textblock. It is very lacking to complete the work with this feature

[Odonno]

Yeah, that's clearly needed.

By the way, I think you should add the suggestion to the Uservoice.

[deltakosh]

I'm working on adding:

• Triple backticks code block
• Images

For videos I don't think this is required here

[deltakosh]

I changed the title to better align with the upcoming PR

[Odonno]

@deltakosh Why not videos? In the use case of documentation for example, people usually use text, code, images and videos.

[deltakosh]

We could add it later. I want to do a first PR with these two. Adding video when images are done will be trivial

[Odonno]

That makes sense.