Anki add-on: Don't remove mark on export

In Anki 2.0.24, the marked tag is no longer kept when you export without scheduling information. Although this is useful to most users (who use marks to indicate that they need to edit the card or want to look at it further themselves), some users may wish to use marking to share information about what cards are more important or otherwise different with other users. Since the change works only on the export—not the import—side, only the user who exports the deck needs to install this add-on, not all users who use the deck.