{pre,post}{Build,Generate}Commands should have optional input and output parameters

[atilaneves]

It's currently impossible to know whether or not one has to run these commands or if they did what they're supposed to since there's no way to know what they do in the general case. With inputs and outputs (including implicit inputs), things get a lot better. It'd also be nice to be able to use something like $in and $out in the commands themselves to avoid repetition. Implicit inputs are files that the command depends on but don't show up in $in.