Twitter support added. Tumblr support dropped.

Twitter support has been added. At the same time, Tumblr support had to
be dropped. In the past, Tumblr used to allow getting information about
posts without registering for an API key. Now, however, it seems that
they started to require one for no reason.

If anyone would like to write code to parse Tumblr's pages, they are
more that welcome. Otherwise, I'm not planning on re-implementing it
myself because their pages are very inconsistent.