Spec: add 'Struct' definition

Worth to add structs with some simple syntax like this:

``` dbml
Struct some_struct {
field_a integer
field_b text
}
```

Reason is PostgreSQL, Oracle DB, IBM DataStage, Ms SQL Server etc sql databases has custom struct type.

I think, that dbml was made not only for sqlite :smile:

`// P.S. i'll try to add more info bit later`