Change validation target for validation plugin

[sitegui]

It would be nice if one could instruct the validate plugin to check something else than an specific argument.

For example:

lifter.use(liftIt.validate({
    valueToCheck: function (req) {
        return req.body
    }
}))
// ...
lifted.run('a/b', req, function () {})

This would check not the first argument, but instead the value returned from valueToCheck.