Speed modifier on Health

This adds a speed modifier when the actors health is above a certain threshold.

For example if you add
```
SpeedOnHealth:
	HPThresholds: 45, 30, 10
	SpeedModOnThresholds: 50, 25, 5
```
to an actor it will fall to 50% speed as soon as its health goes below 45
25% speed when below 30 etc. 
That second line in the suggested desc would be better as `The two values/keys should have the same amount of integers`, no need to get overly technical IMO.