Enhanced update methods

[tejainece]

Implement:

• updateOnly
• updateFieldsById
• updateFields:

  Future<int> updateOnly(User model, Set<String> fields) async {
    // TODO
  }

  Future<int> updateFieldsById(String id, Map<String, dynamic> values) async {
    // TODO
  }

  Future<int> updateFields(
      Expression where, Map<String, dynamic> values) async {
    final st = updater.where(where);
    for (String key in values.keys) {
      final f = fields[key];
      if (f == null) throw Exception('Unknown field!');
      st.set(f, values[key]);
    }
    return execUpdate(st);
  }

[tejainece]

Blocked by #29