Make .Values read-only on CommandArgument/Option

[natemcmaster]

Exposing a raw List of values on CommandArgument and
CommandOption has made it difficult to cleanly implement features like
default values or parsing. This changes the return type of .Values to a
read-only list, and adds TryParse for adding new values, and Reset for
clearing them.

Upgrading

Before

var myOption = new CommandOption("--abc <ABC>", CommandOptionType.MultipleValue);
myOption.Values.Add("xyz");
myOption.Values.AddRange(new [] { "xyz", "123" });
myOption.Values.Clear();

var myArg = new CommandArgument("ABC");
myArg.Values.Add("xyz");
myArg.Values.AddRange(new [] { "xyz", "123" });
myArg.Values.Clear();

After

var myOption = new CommandOption("--abc <ABC>", CommandOptionType.MultipleValue);
myOption.TryParse("xyz");  // note! You should check to see the return type on `TryParse` as it might be values if this value was not accepted.
foreach (var v in new [] { "xyz", "123" })
   myOption.TryParse(v);
myOption.Reset();

var myArg = new CommandArgument("ABC");
myArg.TryParse("xyz");  // note! You should check to see the return type on `TryParse` as it might be values if this value was not accepted.
foreach (var v in new [] { "xyz", "123" })
   myArg.TryParse(v);
myArg.Reset();