Handle underline in text/enriched

mbox2web

@@ -477,6 +477,10 @@ class TextEnrichedPart:
                 # HTML used to have a "big" element, but that has been removed from HTML5
                 pre = "<span style='font-size: 1.1em'>"
                 post = "</span>"
+            elif self.type == "underline":
+                # HTML5 redefined the meaning of "u", but I'm using it anyway
+                pre = "<u>"
+                post = "</u>"
             else:
                 raise NotImplementedError("Unknown type " + self.type)