Add missingFiles option to skip checking for config file existence

[elliottwilliams]

In #220 (review), @yonaskolb suggested that XcodeGen could have a validation setting to disable checking for the existence of config files:

We could create a followup PR that disables missingFiles

I'm playing with using XcodeGen from a context where I don't have config files available, so I went ahead and added a missingFiles option to handle this use case.

One question I have is whether missingFiles should imply missingConfigs, or whether you think it makes sense for those two options to remain distinct. Let me know what you think!

[yonaskolb]

Thanks for the PR @elliottwilliams!
Perhaps missingFiles is a bit misleading as there are other sorts of files that are required and can cause validation errors, for example source files and dependencies)]. For this case, perhaps it should be called missingConfigFiles. In terms of the existing missingConfigs that is seperate and relates to actual config names.

Could you please add a changelog entry as well?

[elliottwilliams]

That makes sense! Renamed to missingConfigFiles.

[yonaskolb]

Great thanks again! One minor thing in the test and then we’re good to go