[FEAT] add #card in first line then create a multiple lines flashcards

[EternalHXP]

Troubled by the current input rules。 If I want to create a multiple lines flashcards. I shoud do:

1. the title of the flashcards
2. ??
3. the back of the flashcards

I think the 2th line is break the alignment of information. I suggest to do like that:

1. the title + #cards
2. the back of the flashcards

perhaps #cards is conflicting, so just add another option to set the tag is OK