Show how to register support for post formats so more people can do it.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@849297 b8457f37-d9ea-0310-8a92-e5e31aec5664
Harden HTML placeholder replacement subroutine, fix `E_NOTICE` message.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@849298 b8457f37-d9ea-0310-8a92-e5e31aec5664
Implement "Content source" field, update README, version bump to 0.3.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@849299 b8457f37-d9ea-0310-8a92-e5e31aec5664