provide a new RichString attribute type
The follwing declaration:
class Card(EntityType):
content = RichString(fulltextindexed=True, default_format='text/rest')
is equivalent to :
class Card(EntityType):
content_format = String(meta=True, internationalizable=True,
default='text/rest', constraints=[format_constraint])
content = String(fulltextindexed=True)