SUG - add an update hook function capability

[demonnic]

Allow user to pass in a function(gauge, current, max) which returns the desired current and max such that current,max = self.updateHook(self, current, max) allows for adjustments to the gauge object itself, or the current or max values to use while templating and setting the gauge value.

Check for nil returns and use original current or max if value nothing returned.

[demonnic]

addressed in https://github.com/demonnic/MDK/releases/tag/v2.8.4