Make options, arguments, and commands *read-only* collections on CommandLineApplication

[natemcmaster]

This gives CommandLineApplication more control over how the definition of the
application is constructed. Validation can be enforced, and refactoring of the internal
storage mechanism can happen without breaking the contract with callers.

Impact

Before

var app = new CommandLineApplication();
app.Options.Add(new CommandOption("-v", CommandOptionType.NoValue));
app.Arguments.Add(new CommandArgument());
app.Commands.Add(new CommandLineApplication());

After

New API has been added to replace these actions

var app = new CommandLineApplication();
app.AddOption(new CommandOption("-v", CommandOptionType.NoValue));
app.AddArgument(new CommandArgument());
app.AddSubcommand(new CommandLineApplication());