Added "decades" tool to the toolset

Thanks for sharing this fun database. I'm learning JS and node.js now, and thought I'd practice a little by adding to this app. I wrote a "decades" tool, so now the user can include --decades=true as an option with the store command. It basically calculates the percentage of each name by decade and outputs the decade percentages as a descending list. It's fun to think of a name as belonging to a certain decade. Mine is definitely a 90's name!