Add defaultConfigurationName to options

This allows users to set the defaultConfigurationName project wide. This
setting corresponds to the drop down in the project settings that says
"Use CONFIG for command line builds". This affects which configuration
Xcode looks in for some settings, even if you pass `-configuration FOO`.

Sources/XcodeGenKit/PBXProjGenerator.swift

@@ -70,11 +70,12 @@ public class PBXProjGenerator {
             )
         }
 
+        let configName = spec.options.defaultConfigurationName ?? buildConfigs.first?.object.name ?? ""
         let buildConfigList = createObject(
             id: spec.name,
             XCConfigurationList(
                 buildConfigurations: buildConfigs.map { $0.reference },
-                defaultConfigurationName: buildConfigs.first?.object.name ?? ""
+                defaultConfigurationName: configName
             )
         )