Replaced item ids in the default config with material names.
Renamed settings 'zero-item' and 'high-zero-item' to
'zero-currency-item' and 'high-zero-currency-item'.
Added optional item name and lore settings for currency and high
currency item (if not set (by default) those will be ignored).
Added optional item data, name and lore for zero currency and high zero
currency items as well (if not set (by default) those will be ignored).
Also values of -1 for item data values will result in the data value to
be ignored.
Slight cleanup in player shopkeeper classes.