added json, xml and txt invoice renderer (#1576)

templates/invoice/renderer/text.txt.twig

@@ -0,0 +1,19 @@
+{% for key, value in model.toArray %}
+{{ key }}:
+{% if value is not empty %}
+{{ value|multiline_indent('    ') }}
+{% endif %}
+
+{% endfor %}
+{% for entry in model.calculator.entries %}
+---
+
+{% set items = model.itemToArray(entry) %}
+{% for key, value in items %}
+{{ key }}:
+{% if value is not empty %}
+{{ value|multiline_indent('    ') }}
+{% endif %}
+
+{% endfor %}
+{% endfor %}