Support meta tags

[VoltanFr]

Some tags should be the union of others. For example, Marine would include Permis bateau maritime and Vocabulaire du marin.

Of course this raises again the question of tag meaning, but this is out of subject here. Make it clear in the doc that this doesn't prevent searching for cards with the tag Marine and without the tag Permis bateau fluvial.

To do:

• card count must take subtags into account
• average must take subtags into account
• search must report cards with subtags

[VoltanFr]

I'm not very sure about this being a good idea. I'm afraid it might be more complicating than helping for users. And the implementation is not easy, with multiple impacts.