Add faction replacements for certain units

[ChrisNeedham24]

Each faction should have one or two units that replace the standard units and provide some difference. Shouldn't be anything major, just an extra stamina point or an extra 5 power/health.

[ChrisNeedham24]

This should maybe wait for some more unit-based enhancements before being done, since there's no real point in adding units with different names for the sake of it. Could wait for #72 or maybe something else?

[ChrisNeedham24]

This is still something that I do want to do eventually, but there will need to be some other wrinkle that's added to the game overall that justifies it, as the previous comment mentions.

[ChrisNeedham24]

Since resources were only added for improvements, that's not something that will make this any more doable. Maybe the faction-specific units could require resources?