Support importing Tumblr

Try to integrate Tumblr's various post types without using
additonal templates.
Fix importing Tumblr photo caption

Besides each photo's caption, the general caption is also needed.

While we're at it, also add a linefeed at the end of file.
Use better titles than None for Tumblr posts without title
Merge pull request #873 from xlz/tumblr-import

Support importing Tumblr