Enhance markdown file preview rendering

[juliushaertl]

This PR tries to apply some sort of styling to markdown previews (mainly giving the headers a different size) and remove markdown symbols that we currently cannot render. This should give the user a bit of an overview how the file is structured.

• Remove markdown symbols that we cannot render (bold/italic/strikethrough/links)
• 4-byte characters are removed since imagettftext is unable to render them even with a ttf font that includes symbols for that, i also gave pango rendering with imagemagick a try for that but none seem to work properly unfortunately

Feedback from @jancborchardt

• Bold headlines
• more space above the headings but less below

[rullzer]

I guess it is waiting for the first report that it doesn't look exactly like the editor. But so be it I guess :)