Added option to show an error message when the user fills in a value not defined in the list:

- ShowErrorMessage: Defines whether to show the error

- CustomErrorTitle: Edit the title of the popup showing when entering an invalid value

- CustomErrorMessage: Edit the text of the popup showing when entering an invalid value

Updating the plugin should not break current use

Added option to define an address string instead of a list of values.

 - This bypasses the hard 255 character limit in Excel for making lists.

- To do so, make the list inside Excel and refer to the location of the new list using the ItemsAddress string.