Allow custom text properties to be included

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

@@ -0,0 +1,15 @@
+import { XmlAttributeComponent } from "file/xml-components";
+
+export interface ICustomPropertyAttributes {
+    readonly fmtid: string;
+    readonly pid: string;
+    readonly name: string;
+}
+
+export class CustomPropertyAttributes extends XmlAttributeComponent<ICustomPropertyAttributes> {
+    protected readonly xmlKeys = {
+        fmtid: "fmtid",
+        pid: "pid",
+        name: "name",
+    };
+}