Overwrite Value Feature for Offline Data Source Feeds

[#1]

When transaction IDs are used and data is import into SiteCatalyst, you cannot change any values assigned to a transaction once a value has already been provided in the past.  If you were to upload new values for a transaction ID that already has values assigned to it, you will simply append the data to existing data for that particular transaction ID. This creates a double count effect.  It would be nice to have a feature that would allow us to automatically overwrite the value currently assigned for a particular transaction with a completely new value.