Generate project via a runner script

[brentleyjones]

Fixes #262 (because what is passed on the command-line now has no affect on the generated project).

This will allow us to move things out of the transition and into xcodeproj.bazelrc. Doing it this way has numerous benefits:

• Changing --features, or other build flags, won't change the Starlark hash, which means the output directory is more stable and has higher cache hits
• We've introduced the rules_xcodeproj* bazel configs (e.g. --config=rules_xcodeproj, --config=rules_xcodeproj_indexbuild, etc.), allowing easy customization of the rules_xcodeproj builds
• We will be able to eventually support user specified --config based customizations (e.g. multiple Xcode configurations)