Update test_date_range to handle flagged data

[wright13]

Consider updating test_date_range to only consider good data and ignore any dates flagged as rejected. Realistically, flags may vary across networks, so maybe provide arguments that let the user specify the name of the flag column and the codes that indicate rejected data

[RobLBaker]

This is a great idea! As a first pass it's worth supporting the "R" flag for rejected. We can work on supporting people who choose not to use the recommended flags at a later stage in development.