exporters follow up

[YousefED]

#1143 ships the first version of pdf / docx exporters. This issue collects possible improvements:

• code block
• fixed table widths for pdf instead of 100% width (difficult, would require a table layout algorithm)
• multi-level bullet list indentation styles (change from solid to open dot to square etc)
• bug: support both strike and underline for same text
• support for audio / video files or preview images
• page break elements