Add remaining Elite Command units

[djwhitt]

See https://github.com/cvincent/elite-command for information on the other Elite Command units. When adding units, create separate issues per unit or for groups of related units and refer to this issue. Additional rules capabilities will also be needed to support air and water units.

[tbeddy]

I've added the land units that don't require any changes to the rules and I'm currently working on the water units.

[tbeddy]

I'm adding the rest of the units in these groups (in this order):

• sniper

• heavy-tank, artillery, heavy-artillery

• cruiser, destroyer, frigate

• bomber, fighter, gunship, mobile-flak

• transport

[tbeddy]

Once one group is merged, I'll make a PR for the next group.

[tbeddy]

I haven't started work on transport, so once all the other units are done, I'll shift my focus to whatever needs doing for the July release.

[tbeddy]

I have started work on transport ( #84 ). I'm aiming to have all Elite Command units ported for the next release.