Fix '{% vm %}' tag for table cells with XML attributes

Updated regex to match cell content inside '<w:t>' even if <w:t> has XML
attributes such as 'xml:space="preserve"'. Updated test to have this kind
of '<w:t>'.

docxtpl/__init__.py

@@ -103,7 +103,7 @@ class DocxTemplate(object):
                     m1.group(4)  # ``</w:t>``.
                 )
             return re.sub(
-                r'(</w:tcPr>.*?<w:t>)(.*?)(?:{%\s*vm\s*%})(.*?)(</w:t>)',
+                r'(</w:tcPr[ >].*?<w:t(?:.*?)>)(.*?)(?:{%\s*vm\s*%})(.*?)(</w:t>)',
                 vMerge,
                 m.group(),  # Everything between ``</w:tc>`` and ``</w:tc>`` with ``{% vm %}`` inside.
                 flags=re.DOTALL,