Add option to set a global default for enabling the Tumblr auto-tweet.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@984902 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme, mark compatibility with WP 4.0, version bump to 0.7.15.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@984903 b8457f37-d9ea-0310-8a92-e5e31aec5664