MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
@zdne So, I wondered about this myself. If we do it, from and AST standpoint, I think we need to keep track of the inline description as being different from a block description, so I think from a parsing standpoint and AST standpoint we should have an inline description section that is an honorary section type much like a mixin is an honorary member type so to say.