Add treeExpandButtonColumnKey option to specify the column where tree button should be shown

#262
example:

const tablePropsInit: ITableProps = {
  columns: [
    { key: 'name', title: 'Name', dataType: DataType.String },
    { key: 'productivity', title: 'Productivity', dataType: DataType.Number },
  ],
  //...
  treeExpandButtonColumnKey: 'productivity',
  //...
};