Rework zotero_keys and zutilo_keys to use ex commands with mappings set to the ex commands. This update should have all of the same mappings as the previous version. It just now has ex commands as well. The next steps will be:

1. to add additional options to some of the commands
2. consolidate some commands into different options of a single command
3. make mappings optional so that they are not forced on the user and the user can choose to map the ex commands to different keys
4. add new commands that take arguments
5. clean up a few bugs (like tagging and focusing Zotero not working on the first try after startup)