Add support for strike through attribute to markup spans

It can be handled in exactly the same way as underlining, so just do it
instead of leaving a "TODO" comment about it.

include/wx/private/markupparserattr.h

@@ -135,7 +135,9 @@ public:
                              font, &wxFont::SetUnderlined,
                              false, true);
 
-        // TODO: No support for strike-through yet.
+        FontModifier<bool>()(spanAttr.m_isStrikethrough,
+                             font, &wxFont::SetStrikethrough,
+                             false, true);
 
         switch ( spanAttr.m_sizeKind )
         {