Expose Option.IsSetDefault

Fixes #213.
diff --git a/option.go b/option.go
index 17a2189..8270a22 100644
--- a/option.go
+++ b/option.go
@@ -179,6 +179,11 @@
 	return option.isSet
 }
 
+// IsSet returns true if option has been set via the default option tag
+func (option *Option) IsSetDefault() bool {
+	return option.isSetDefault
+}
+
 // Set the value of an option to the specified value. An error will be returned
 // if the specified value could not be converted to the corresponding option
 // value type.