feature: Add HeaderImage attribute.

[shanecelis]

Hi Denis,

Here is my patch to add a HeaderImage attribute. It's based on the ShowAssetPreview code. It works like this:

public class _NaughtyComponent : MonoBehaviour
{
  public const string ICON
    = "Assets/NaughtyAttributes/Samples/DemoScene/TestAssets/quadratic.png";
  [HeaderImage(ICON)]
  public float a;

  [HeaderImage(ICON, Height = 96, Alignment = EAlignment.Center)]
  public float b;

  [HeaderImage(ICON, Width = 96, Alignment = EAlignment.Right)]
  public float c;
}

The README has been updated with essentially what's shown above.

I have tried to follow suit with the existing code base. If I have violated on any conventions, I apologize and am happy to see them corrected.

Thank you for the excellent NaughtyAttributes project. I hope this is helpful.

-Shane
twitter: @shanecelis

[dbrizov]

This is for the older version. I created a request, and will implement it when I can. #271