Add action: Alter item quantity

Great module, extremely useful rules, handy even without using the standard stock side of things (I needed a Max allowed order for items with unlimited stock, rules provided allowed for it quite easily).

I did have a request from the client though, they wanted instead of preventing the order and showing an error, to have the quantity of the particular item to be automatically dropped down to the highest amount available and a nice message displayed saying as much.

I couldn't see anyway of doing this, and thought it would be a very handy addition, in the case of standard stock you could have it reduce it to the maximum amount of stock left over.