create a enum to define length of MD5 hash.

Author: catcherwong

src/NETCore.Encrypt/Internal/MD5Length.cs

@@ -0,0 +1,8 @@
+namespace NETCore.Encrypt
+{
+    public enum MD5Length
+    {
+        L16 = 16,
+        L32 = 32
+    }
+}