If you'd like a javascript version (through xkit), because adding before & after quotation marks depends on having paragraphs:

1. have new-xkit installed in your browser and go to www.tumblr.com/xkit_editor
2. click "New Extension"
3. copy-paste the code from http://pastebin.com/kkvyq5C0 into the "Script" tab
4. copy-paste the code from http://pastebin.com/kSQjgNCn into the "Stylesheet" tab
5. save & profit