Added option 'position' to a column

This functionality is useful when you have a datagrid and want to have another datagrid with one more column, but added to a position other that last. like in this case:

```ruby
class Report
  include Datagrid
  scope do
    SomeModel
  end
  column(:name)
  column(:description)
end

class InheritedReport < Report
  column(:id, :position => 1)
end
```

Without this option, it would be impossible to add the id column between the name and the description.
Note, that this does not if you try to position a column in a position thats not already defined. So this wouldn't work:

```ruby
class Report
  include Datagrid
  scope do
    SomeModel
  end
  column(:name, :position => 1)
  column(:description)
end
```

I could change the code a little bit to support that, if it's needed.