Added Hive.ignoreTypeId method

[themisir]

The method adds support for ignoring not used types. It simply creates an empty IgnoredTypeAdapter for given type id.

Fixes #332

class IgnoredTypeAdapter<T> implements TypeAdapter<T> {
  IgnoredTypeAdapter([this._typeId = 0]);

  final int _typeId;

  @override
  T read(BinaryReader reader) {
    return null;
  }

  @override
  int get typeId => _typeId;

  @override
  void write(BinaryWriter writer, obj) {}
}

Example:

Hive.ignoreTypeId(2);