Add optional 7-day smoothing window

[scottgigante]

Here's a simple implementation of a 7-day smoothing window using a checkbox on the package page.

I haven't been able to debug this as it's not clear to me how to run the app. Closes #21.

[crflynn]

I've just updated pypistats, and it should be easy to run locally using docker compose if you merge the changes.

[scottgigante]

@crflynn this works pretty well in my tests now. Thanks for making it easy to contribute!