create if/elif/else syntax for attributes
The proposed syntax
```yaml
seq:
  - id: a
    type: u1
  - id: b
     type: u8
instances:
   c:
     value:
       if:
         'b>0': b
         'b<=0 and a>10': a
          _: 10
```

Yes, I know that a quite non-standard parser preserving the order of items is required. But I don't consider it as a big problem.