Refactor theme support detection and adding.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@1055922 b8457f37-d9ea-0310-8a92-e5e31aec5664
Closes #15: Uses a featured image if no `<img>` is found in post body.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@1055923 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update changelog in readme, version bump to 0.7.19.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@1055924 b8457f37-d9ea-0310-8a92-e5e31aec5664