refactor: default to imports subdoc and ignore importerror

Author: waketzheng

docxtpl/__init__.py

@@ -12,3 +12,7 @@
 from .listing import Listing
 from .richtext import RichText, R, RichTextParagraph, RP
 from .template import DocxTemplate
+try:
+    from .subdoc import Subdoc
+except ImportError:
+    ...