todo:

  ☐ add **custom css** feature
  ☐ sync scroll @needsUpdate
  ☐ load images from internet (`https:`)
  ☐ regive focus to the right markdown view
  ☐ set the title of the preview
  ☐ disable previewing when the preview is closed
  ☐ preview.set_scratch(True)
  ☐ add 404 image