Add helper attribute to ignore arguments

[ModProg]

This is related to e.g. #67, but for the simple usecase of just wanting to ignore one or two arguments, something like

#[cached]
fn hello(a: usize, #[cached(skip)] b: String) 

would be nicer to use IMO.

[ewoolsey]

+1. Current method is pretty clunky, especially when you've got a lot of other parameters.