Add: PROC

[iaadefila]

Why this feature is better to be included in stock-pandas? Please describe the scenario

Through a simple calculation, the inclusion of Price Rate of Change (PROC) will further enhance the applicability of the library as a one-stop for stock analysis.

Provide 1-2 code examples that the usage of the new feature:

Are you willing to submit a pull request to implement this rule?

[kaelzhang]

ROC (or PROC) is equivalent to

stock['change:(column:close),20']

# 20 means the 20-period ROC
# calculate on close prices

Check https://github.com/kaelzhang/stock-pandas#change for details

For now, you could use this to have a try