Cards injected using markdown2anki should have a md2anki tag for future use

[Mochitto]

In the future, there might be the need to fetch all cards created using markdown2anki (editing them, statistics etc.). An easy way to provide an "handle" to do this, is to tag said cards starting from now, even if we don't yet know what we might do with cards that have already been created.

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This should be done automatically, without the option to turn it on or off. It should be easy to add after having implemented #3 and #11, as the tag can just be pushed with the others given by the user.