`purchase` paid apps

Purchase paid (non-free) apps.

Maybe add a --purchase flag to mas install after #557 is resolved.

If someone tries to install a non-free app that wasn't previously purchased without including --purchase, output an error specifying the cost and the necessity to supply the --purchase flag.

To ensure that people don't spend more than they expect on an app, we could require user to specify a maximum cost they're willing to pay (like --purchase 10.00USD with a special --purchase unlimited value to specify no limit). We'd have to check how the store handles prices in different currencies. We would require that the specified currency match the currency expected by the MAS for the signed-in account for the purchase of the specified app.

Start by reviewing the dependency on issue #557 and the existing mas install command. Investigate how the Mac App Store reports prices and account currencies, then define the purchase and maximum-cost behavior, including errors when --purchase is absent. Done means paid apps can be purchased safely with the specified limit and currency requirements.