Add contentMode as an option to remote_image

Some of my images where stretched for a remote_image cell, so I added the content_mode.

Is this something you would like in the ProMotion repo? :)

```ruby
def table_data
  @table_data = [{
    cells: [
      {
        remote_image: {
          url: "http://placekitten.com/200/300",
          placeholder: 'placeholder.png',
          content_mode: UIViewContentModeScaleAspectFit
        }
      }
    ]
  }]
end
```