How to use custom Authorize attribute for roles as well as a specific user?

You could write a custom authorize attribute:

public class AuthorizeAdminOrOwnerOfPostAttribute : AuthorizeAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        var authorized = base.AuthorizeCore(httpContext);
        if (!authorized)
        {
            // The user is not authenticated
            return false;
        }

        var user = httpContext.User;
        if (user.IsInRole("Admin"))
        {
            // Administrator => let him in
            return true;
        }

        var rd = httpContext.Request.RequestContext.RouteData;
        var id = rd.Values["id"] as string;
        if (string.IsNullOrEmpty(id))
        {
            // No id was specified => we do not allow access
            return false;
        }

        return IsOwnerOfPost(user.Identity.Name, id);
    }

    private bool IsOwnerOfPost(string username, string postId)
    {
        // TODO: you know what to do here
        throw new NotImplementedException();
    }
}

and then decorate your controller action with it:

[AuthorizeAdminOrOwnerOfPost]
public ActionResult EditPosts(int id)
{
    return View();
}