Allow custom text properties to be included

src/file/custom-properties/custom-properties-attributes.ts

@@ -0,0 +1,13 @@
+import { XmlAttributeComponent } from "file/xml-components";
+
+export interface ICustomPropertiesAttributes {
+    readonly xmlns: string;
+    readonly vt: string;
+}
+
+export class CustomPropertiesAttributes extends XmlAttributeComponent<ICustomPropertiesAttributes> {
+    protected readonly xmlKeys = {
+        xmlns: "xmlns",
+        vt: "xmlns:vt",
+    };
+}