Add 'Power Rankings' tab

[earlejam]

In addition to the season-long, unweighted "Summary" tab which keeps track of the expected wins of each team, it would be nice to have a separate "Power Rankings" tab which applies some weighting to the expected wins of each team.

The simplest form could be applying some weighted multiplier which favors a team's recent performance and considers earlier weeks less heavily. Without doing any research, this could be some linear scale or a bell-curve like weighting.

A more advanced version might consider when teams have key players on bye, or more heavily weight a team's performance after a trade (i.e. completely different roster than earlier in season).