Add prompt if `zet.public` not found

[oglinuk]

Added a check using the -d bash conditional expression in the _initialize() function. If ${CONF[zet.public]} does not exist, then tell user to create the directory, or change the value of zet.public. Unsure if the _initialize() function is the correct place for the check, and also unsure if wanting user to manually create directory or have zet do it. Keen to hear your thoughts @rwxrob. This resolves issue #39.