Adding embeddable cards in a backward compatible way.

[vavdoshka]

Added embeddable cards in a backward-compatible way. Allows limiting the scope of the processing to the content between <!-- BEGIN_ANKI_CARDS --> and <!-- END_ANKI_CARDS --> comments.

I found it helpful to embed the Anki Cards definitions into documents with some knowledge base data. This change is backward compatible and should work with all the existing features from how far I've tested.

[jasonwilliams]

Hey @vavdoshka thanks for this PR, looks good. I’ll try and take a proper look

[vavdoshka]

I just updated the logic slightly.

[jasonwilliams]

Hey @vavdoshka, Implementation looks good, it may be worth adding a test here or here

[jasonwilliams]

Pings @vavdoshka are you still active on this?