Add support for frequent flyer programs

It would be really useful -- since you know how far I'm traveling and which airlines I'm on -- to track mileage program numbers... Every now and then I get emails telling me how many miles I've accrued, but it'd be smart for TripIt to do this. Expedia (I believe) offers a similar feature, but theirs is useless.

If you both counted the numbers of miles I've accrued based on YOUR estimates, then let me forward you my mileage emails with THEIR counts, it'd be fascinating to see if the airlines are actually accurate in their calculations.