Add enhancements to progress widget

* Adds TransferSpeedColumn
* Adds DownloadedColumn
* Adds ElapsedTimeColumn
* Minor enhancements to existing columns

examples/Console/Progress/Program.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Threading;
 using Spectre.Console;