Add --project-directory option to generate command to easily use relative path

Sources/ResgenSwift/Generate/GenerateOptions.swift

@@ -16,4 +16,13 @@ struct GenerateOptions: ParsableArguments {
     
     @Argument(help: "Configuration file.", transform: { $0.replaceTiltWithHomeDirectoryPath() })
     var configurationFile: String
+    
+    @Option(help: "Project directory. It will be added to every relative path (path that does not start with `/`",
+            transform: {
+        if $0.last == "/" {
+            return $0
+        }
+        return $0 + "/"
+    })
+    var projectDirectory: String
 }