Hybrid engines (WIP)

Lets the driver toggle a 'hybrid mode' from the vehicle controls when an electric and (diesel or gasoline) engine is present. In this mode, defaults to electric engine off and combustion engine on. Vehicle controls then let you switch to the electric engine, turning off any combustion engines. Automatic switch from electric to combustion when battery level is low enough, or vehicle exceeds safe velocity. Manual switch required for turning on electric engines.
Still a bit buggy though, fuel usage doesn't update for engines that are off. 
I figure mechanics 12 is required for installing two engines so it should be balanced-ish, but a part could always be required to enable this stuff.
Minor nitpick -- from the indentation here, it looks like the || is applying on the same level as combustion_engine_on, but it's inside parentheses. Also, my vague recollections from the style guide suggest that the boolean operator should be going on the end of the previous line.

Consider:
```C++
if(combustion_engine_on && (part_info(engines[p]).fuel_type == fuel_type_gasoline || 
                            part_info(engines[p]).fuel_type == fuel_type_diesel)) {
```