Feature Deck Configurations

Adding support for deck configurations. Now there is a new class called DeckConf that contains a dictionary with the deck configuration (with exactly the same nomenclature as used in the Anki database). The Deck class now has a new parameter called conf that expects a DeckConf object.

Notes:

• If we reuse the same configuration in several decks, we are really doing 2 updates in the database when one would be enough.
• For the configuration to be imported there must be at least one new card in the collection. This is a limitation of the Anki library, not of the apkgs generator. More details: https://anki.tenderapp.com/discussions/ankidesktop/38114-importing-apkg-does-not-update-deck-config-fields

I know there is another pull request with related content. I just want to share what I did to meet the needs of my project.