Modify project plist for decorated icons during build time

Hi,

iOS-Build-Kit is nice especially for the decorated icons feature.
However, original design of the usage is to add decorated icons in project plist.
To fulfill both these cases, the manual settings at project plist might not be a good idea.

1. Formal build from buildkit with decorated icon
2. Testing build from Xcode GUI without decorated icon, 

Sometimes I will get a empty app icon due to decorated icon did not generated in Xcode GUI.

The PR is to modify the strategy that modify project plist with the decorated icons at build time.
Please help to see if this change makes sense.

Forgive that I am not quite familiar with Ruby, and I did not update the document and example project.
If there are any further changes need to do, please let me know.
Thanks and looks forward to seeing iOS-Build-Kit to be better and better.
Kudo