Add new unit - Bomb Airship
1. Flying unit
2. Compared with the bomb aircraft, more HP, higher DEF, faster atk, slower move speed and smaller range
3. Not appear in regular waves, appears in random waves
4. Combinable
Added new item - Oil, Rubber, Oil Drill, Oil Refiner
1. A series of requirements for the combination of bomb planes
2. Balloon units can now combinable