Add CLI parameter for running without graphics

[tslocum]

Resolves #1547.

[TheJJ]

thanks! an improvement idea: there may be more modes in the future, so instead of single flags, i think having a --mode=headless or --mode=full would be more future-proof :)

[heinezen]

Looks good to me :)

The only thing missing is that you have to add your info to https://github.com/SFTtech/openage/blob/master/copying.md . Otherwise it'd be good to go.

You can try to implement @TheJJ 's suggestion. That would be a bit more tricky but more future proof if we add more modes.

[tslocum]

I have added my info. I agree that expanding this is more future-proof, but will leave that to someone else when the need arises. Thank you both.

[heinezen]

@tslocum Alright :)

Your info doesn't match up with the commit mail, so please change that and we can merge.

[heinezen]

Also maybe do a rebase fro master while you are at it :D

[tslocum]

Gladly. I think this is all fixed up now.