Add moving average prices to ```generate_labels.py````

[kamilkrukowski]

We want to add several columns for downstream model analysis

1. Stock Price at opening the day AFTER 8-K release
2. N-day moving average centered at 7 days after 8-K release
3. N-day M.A. centered at 30 days after 8-K release
4. N-day M.A. 90 days after release

Ideally we want N to be a hyper-parameter that is a constant set at the top of the file.