Feature: Add limit when using price indexing

[sgornick]

When I place an order using price indexing (e.g., {mtgoxbid} ) I might want to only offer up to (or down to) a certain limit.

e.g.. I want to place a bid that is indexed to the mtgoxbod, but only up to $1.25

Currently I can do this in the notes, but it would seem to be not terribly complicated to add the ability to set an upper or lower bound.

e.g., ;;buy 100 BTC @ "max({mtgoxbid}, 1.25)" USD Can pay using Dwolla