Allow datetime filtering options

[jonhealy1]

Presently we only search on the datetime field unless an item has a null datetime in which case, we search on start_datetime and end_datetime. We should look at the option of including other filtering behaviors such as:

• Always include start_datetime and end_datetime in a search. (previous api behavior)
• Never use the start_datetime and end_datetime fields.

Options could be set via environment variables and documented in the readme.