Add 'item.default' option

If 'item.default' key is specified in configuration entry, and
'item.client' list is not specified or is empty, string specified in
'item.default' will be used instead of the client list. You can use this
option to allow connections from any host automatically (by specifying
'ALL').
Merge pull request #7 from drybjed/default

Add 'item.default' option