add support --version option

[mironal]

I think that it is better for xcodegen to have --version option as it is for many commands.

However, I think that this will increase the effort of editing main.swift for version up process. 😟
If you feel troublesome, you can reject this pull request. 😇

[keith]

This would be fantastic! @yonaskolb thoughts on the extra work for shipping?

[yonaskolb]

Yes, adding this is definitely a good idea. Thanks @mironal!
In terms of keeping this updated I think the version should live in one place and then a release script would update it places like this, tag a version, and also use it to update the brew formula.
The release script can live in the make file and the version could either be in there or in it's own file. Is there a standard way of doing that?

[yonaskolb]

Would this --version flag or a xcodegen version command be preferred?

[keith]

I think we should just bake that in to a build script somewhere, to sed out a variable in a swift file with a version number. Personally I prefer --version but don't feel strongly