Add support for NULL/NOT NULL-based sorting (#49)

Sources/SQL/SQLDirection.swift

@@ -2,9 +2,15 @@
 public protocol SQLDirection: SQLSerializable {
     /// Ascending order.
     static var ascending: Self { get }
-    
+
     /// Descending order.
     static var descending: Self { get }
+
+    /// Order in which NULL values come first.
+    static var null: Self { get }
+
+    /// Order in which NOT NULL values come first.
+    static var notNull: Self { get }
 }
 
 // MARK: Generic
@@ -15,23 +21,41 @@ public enum GenericSQLDirection: SQLDirection {
     public static var ascending: GenericSQLDirection {
         return ._ascending
     }
-    
+
     /// See `SQLDirection`.
     public static var descending: GenericSQLDirection {
         return ._descending
     }
-
+
+    /// See `SQLDirection`.
+    public static var null: GenericSQLDirection {
+        return ._null
+    }
+
+    /// See `SQLDirection`.
+    public static var notNull: GenericSQLDirection {
+        return ._notNull
+    }
+
     /// See `SQLDirection`.
     case _ascending
-    
+
     /// See `SQLDirection`.
     case _descending
-
+
+    /// See `SQLDirection`.
+    case _null
+
+    /// See `SQLDirection`.
+    case _notNull
+
     /// See `SQLSerializable`.
     public func serialize(_ binds: inout [Encodable]) -> String {
         switch self {
         case ._ascending: return "ASC"
         case ._descending: return "DESC"
+        case ._null: return "IS NULL"
+        case ._notNull: return "IS NOT NULL"
         }
     }
 }