Add async custom processing support. Add chainArgParserCalls() for configuration. Additionally await thenable implicit and default option values and thenable default argument values

lib/argument.js

@@ -16,6 +16,7 @@ class Argument {
     this.description = description || '';
     this.variadic = false;
     this.parseArg = undefined;
+    this.chained = true;
     this.defaultValue = undefined;
     this.defaultValueDescription = undefined;
     this.argChoices = undefined;
@@ -89,6 +90,17 @@ class Argument {
     return this;
   }
 
+  /**
+   * When set to `true` (the default), next call to the function provided via `.argParser()` will be chained to its return value if it is thenable.
+   *
+   * @param {boolean} [chained=true]
+   * @return {Argument}
+   */
+  chainArgParserCalls(chained = true) {
+    this.chained = !!chained;
+    return this;
+  }
+
   /**
    * Only allow argument value to be one of choices.
    *