Fix #103: Make image zoom toggle keyboard accessible

When navigating directly to an image, and the image does not fit the
screen, it is re-sized so it does. The mouse cursor becomes a magnifying
glass when hovering the image. By clicking you can toggle between native
and re-sized.

As discussed in #355, to keep things simple we now make such images
markable. Using the default mappings and hintchars, the shortcut to
toggle zoom then becomes 'ff'.

This commit is based on #104 by @LordJZ.

Example image:
<http://upload.wikimedia.org/wikipedia/commons/e/e2/Rain_ot_ocean_beach.jpg>