Refactored bash complete script to have one return statement.
Updated tag bash completion to include opts and the first arg to return credentials.
Updated tags table to disallow empty tags, require unique tags, and disallow NULL tags.
Updated passwords and usernames tables to abort if username or password is not unique.
Enabled foreign key support for sqlite3 when opening safeword database.
Updated tagged_credentials table to cascade on delete.
Removed debug statement.
Moved delete_tag() and rename_tag() from TagCommand to safeword as library functions.
Implemented --move option for tag command. Refactored TagCommand to use subcommand structure.