Show the extra separators below UITableView

By default on iOS, UITableView shows extra separators in the empty place below the cells:


As `RNTableView` is always initialising `tableFooterView`, these separators aren't shown. Could we initialise the footer only when we actually need?

At the moment, you can disable the footer by **commenting** this line:
```objc
_tableView.tableFooterView = [[UIView alloc] initWithFrame:CGRectZero];
```

And this line:
```objc
_tableView.tableFooterView = view;
```