Allow hexadecimal notation for enum values

It would be beneficial to allow encoding of the enum value
in hexadecimal, example:

```
enum {
    value 0x1337;
}
```

This is not allowed in YANG 1.1. Howere, default integer
values are allowed to be encoded  in hexadecimal.

The use case where I want it is in protocol version YANG
modules, where the protocol version is definen in an
IANA registry with a hexadecimal value.