date_preposition()

The date_preposition() function takes a date string and returns the
proper preposition to precede the date in a sentence (e.g., "in" 2014,
"on" March 14, "" next year). This function was written in the context
of the type of date arguments represented in Wavii's data. I taught
myself the Python used here via Codecademy, and learned the regex
syntax from a co-worker.