JoinDict: add spec of "validator".
JoinDict: add get_all_keys() and get_all_values() methods.
JoinDict: change the spec of "filter". Now that get_all_keys() and get_all_values() are available, only $dict is necessary in the filter code-ref.
JoinDict: implement the validator option.
JoinDict: POD: add comments about filter and validator inheritance.