Add min_periods to pd.rolling in rolling_averages function

Currently, the rolling average calculation does not have a min_period to calculate a mean, so at the beginning of a dataset, a 7-day, 14-day, or 28-day average will not be a mean of n-days but <n-days until the nth day. We should add a min_period to the rolling_average function, but it will need to parse a string into a number (e.g. user inputs '28-day' and we need to extract the integer 28 to be placed into the min_period argument).

See: #40 (comment 537732)