Introduce attack strategies

[nvs]

There could be an issue already about this, I cannot remember.

Anyways, the idea being that all towers could have access to a variety of attack strategies. This would assist in reducing the micro requirement within the game (e.g. with DoTs).

Some ideas:

• Nearest target
• Farthest target
• Lowest HP target
• Highest HP target
• Hold fire
• Hit different target each hit