Add support for per-xcodeproj Bazel configurations

[brentleyjones]

Fixes #641.

[jpsim]

The way this works is a bit surprising to me.

For example I expected that if I have this config:

build:my_config --define=abcd=1

and I set config = "my_config", in my xcodeproj invocation that the --define would be picked up when building in Xcode, but in fact I have to set this in my config:

build:my_config_build --define=abcd=1

Previously if I set something on the rules_xcodeproj config in bazelrc, it would propagate to all rules_xcodeproj specialized configs (build, info, etc.).

Should this follow the same principle?

[brentleyjones]

Just build:my_config --define=abcd=1 should work, as build:my_config_build --config=my_config should be in the xcodeproj_extra_flags.bazelrc. I'll see why that's not the case.