tumblr_backup: add option -k to disable image download

Image downloading is space and time consuming; since Tumblr always keeps
pictures on its servers even after a blog's deletion, bypassing the
image saving process would allow users to quickly keep the skeleton of
their blogs. The backup would then load the missing images directly from
Tumblr servers.
tumblr_backup: cleanup the previous commit (fixes #23)
Merge branch 'gh-23'