title: Asp.net Identity 2.0 extend UserValidator with custom unique property
body: <p>I would like to extend UserValidator or something similar in Asp.net Identity 2.0 to not only check for unique email but also a unique value of my choosing. Example of what I would like to do with Alias below. Is this possible or do I have to write a check everywhere I can update Alias?</p>
  
  <pre><code>public class ApplicationUserManager : UserManager&lt;ApplicationUser, int&gt;
  {
      public ApplicationUserManager(IUserStore&lt;ApplicationUser, int&gt; store)
          : base(store)
      {
          this.UserValidator = new UserValidator&lt;ApplicationUser, int&gt;(this)
          {
              AllowOnlyAlphanumericUserNames = false,
              RequireUniqueEmail = true
              //This does not exist
              //RequireUniqueAlias = true
          };
      }
  }
  
  public class ApplicationUser : IdentityUser&lt;int, CustomUserLogin, CustomUserRole,
  CustomUserClaim&gt;
  {
      [Required]
      public string Alias { get; set; }
  }
  </code></pre>