improved invoices with new renderer (#306)

* added docx renderer and demo template
* added csv renderer and demo template
* added xlsx renderer and demo template
* added ods renderer and demo template
* added user calculator
* added invoice documentation

src/Invoice/NumberGeneratorInterface.php

@@ -25,4 +25,14 @@ interface NumberGeneratorInterface
      * @return string
      */
     public function getInvoiceNumber(): string;
+
+    /**
+     * Returns the unique ID of this number generator.
+     *
+     * Prefix it with your company name followed by a hyphen (e.g. "acme-"),
+     * if this is a third-party generator.
+     *
+     * @return string
+     */
+    public function getId(): string;
 }