Fix `eww-current-source' buffer confustion

* net/eww.el (eww-display-html): Set `eww-current-source' in the
correct buffer.
(eww-view-source): Use it.
Add a new, somewhat experimental "readability" command to eww

* net/eww.el (eww-readable): New command and keystroke.

* net/shr.el (shr-retransform-dom): New function.