Save `use_excerpt` even if not crossposting, genericize settings names.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@882085 b8457f37-d9ea-0310-8a92-e5e31aec5664
Genericize remainder of plugin settings names, version bump to 0.7.8.5.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@882087 b8457f37-d9ea-0310-8a92-e5e31aec5664