Add a memoization/caching option

[Paethon]

It would be really great if there would be an option for the @manipulate macro to turn on memoizing/caching of the results. Especially when using complicated plotting commands, everything gets pretty unresponsive.

A manual solution which works is to use to the Memoize.jl package, put everything executed by @manipulate in a function and define it using the @memoize macro. But it would be great if this could be done automatically.