delete invoices (#1652)

* include all meta fields as template variables
* support invoice preview via command
* support deletion of generated invoices

tests/Invoice/Renderer/DebugRendererTest.php

@@ -84,6 +84,7 @@ class DebugRendererTest extends TestCase
             'invoice.currency_symbol',
             'invoice.vat',
             'invoice.tax',
+            'invoice.language',
             'invoice.tax_nc',
             'invoice.tax_plain',
             'invoice.total_time',
@@ -109,6 +110,14 @@ class DebugRendererTest extends TestCase
             'query.month',
             'query.month_number',
             'query.year',
+            'query.begin_day',
+            'query.begin_month',
+            'query.begin_month_number',
+            'query.begin_year',
+            'query.end_day',
+            'query.end_month',
+            'query.end_month_number',
+            'query.end_year',
             'customer.id',
             'customer.address',
             'customer.name',