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/ProjectSpec/ProjectSpec.swift

@@ -32,6 +32,7 @@ public struct ProjectSpec {
         public var indentWidth: UInt?
         public var xcodeVersion: String?
         public var deploymentTarget: DeploymentTarget
+        public var defaultConfigurationName: String?
 
         public enum SettingPresets: String {
             case all
@@ -66,7 +67,8 @@ public struct ProjectSpec {
             usesTabs: Bool? = nil,
             xcodeVersion: String? = nil,
             deploymentTarget: DeploymentTarget = .init(),
-            disabledValidations: [ValidationType] = []
+            disabledValidations: [ValidationType] = [],
+            defaultConfigurationName: String? = nil
         ) {
             self.carthageBuildPath = carthageBuildPath
             self.carthageExecutablePath = carthageExecutablePath
@@ -80,6 +82,7 @@ public struct ProjectSpec {
             self.xcodeVersion = xcodeVersion
             self.deploymentTarget = deploymentTarget
             self.disabledValidations = disabledValidations
+            self.defaultConfigurationName = defaultConfigurationName
         }
 
         public static func == (lhs: ProjectSpec.Options, rhs: ProjectSpec.Options) -> Bool {
@@ -217,5 +220,6 @@ extension ProjectSpec.Options: JSONObjectConvertible {
         tabWidth = (jsonDictionary.json(atKeyPath: "tabWidth") as Int?).flatMap(UInt.init)
         deploymentTarget = jsonDictionary.json(atKeyPath: "deploymentTarget") ?? DeploymentTarget()
         disabledValidations = jsonDictionary.json(atKeyPath: "disabledValidations") ?? []
+        defaultConfigurationName = jsonDictionary.json(atKeyPath: "defaultConfigurationName")
     }
 }