Bugfix: escape markdown control chars in markdown output of tweet text body

[flauschzelle]

In the text body of tweets (for markdown output):

• Escape possible markdown control characters (any of these: \_*[]()~`>#+-=|{}.!)
• Add double spaces before line breaks

So that the rendering of the tweet text looks like the original tweet text - including line breaks and characters that could otherwise be hidden and/or misinterpreted by markdown rendering.

Fixes #40

I could (and would like to) also do this for the markdown output of DMs (single and group).

But because it needs (a little bit of) special handling for the media embedding, I would wait for #116 and #118 to be merged before I apply this to DMs.