Using `dataItemColumnValueExtractor` function to extract value from map

Better approach: realized by convert map to object and overwrite operator[]

Sample Code

/// when data is hierarchical map
///  data:  {
///    'start': {'a':"01/01/200$i", 'b':'ccc'},
///    'end': 'xxx'
///  })
  grid.GridOptions opt = new grid.GridOptions()..dataItemColumnValueExtractor=mapExtract;
  grid.SlickGrid sg= new grid.SlickGrid.fromOpt(el,data,column,opt);
  /// extractor function example
  mapExtract(Map row, grid.Column col){
    if(col.field=='start') return row['start']['a'];
    return row[col.field];
  }