todo:

  ☐ sync scroll @needsUpdate(because of images)
  ✔ load images from internet (`https:`) @done (17-01-02 15:51)
  ☐ regive focus to the right markdown view
  ☐ set the title of the preview
  ☐ preview.set_scratch(True)
  ☐ add 404 image
  ☐ fix #4
  ☐ bug when empty `src`
  ☐ auto refresh preview if loading images
  ☐ call settings listener on_new too
  ☐ preview.wordWrap => True