Support nested data

[AllenFang]

Implement #14

it's able to configure the dataField to access the nested data,

const columns = [{
  dataField: 'id',
  text: 'Person ID'
}, {
  dataField: 'nest.address.road',
  text: 'Road Name'
}, {
  dataField: 'nest.postal',
  text: 'Postal'
}];

Issue related with AllenFang/react-bootstrap-table#50
react-bootstrap-table2 will start support nested data from v0.0.1.