I understand this isn't exactly what you're looking for, but it does the same thing aside from turning the water flow on and off. On my heater, you had one main temperature control which would be located next to the heater or in a utility closet. The control I linked to is an auxiliary unit specifically designed for use in a bathroom. When turned on (taking a shower), it takes precedence over the main controller which is likely set to 120º all the time. When done showering, you turn it off and the main controller takes back over.

On my particular water heater, you could have 3 total temperature controllers. One main, and two auxiliary.

You might be looking for something more like this which doesn't appear to require a new water heater.