Game mechanics - Difficulty scaling

[Seblis]

Every 30 (?) seconds we can up difficulty a bit. Possible changes include (list to be adjusted):

• increased spawn rate
• increased enemy hp
• increased enemy speed
• increased enemy damage

Single difficulty spike doesn't have to include all of the above - most like only one thing at a time.

[Seblis]

Since the deadline is too soon, for now we'll use simplified difficulty scaling that will be described in another issue