Add function to return CFDE funding opportunity numbers #16

The CFDE includes a list of funding opportunities numbers:

https://commonfund.nih.gov/dataecosystem/FundingOpportunities

The opportunity numbers can then be used in the RePORTER API (or website--see image) to identify all grants that have been funded under those opportunity numbers. The function should return the opportunity numbers in a form that can be used for filtering. Note that this function is not generalizable and is meant to serve the CFDE reporting effort only.