Create mechanism for late addition of Command in CommandBuilder

[KathleenDollard]

Currently the Command for CommandBuilder is added in the constructor. If none is defined for CommandBuilder, an exception occurs. For CommandLineBuilder, a default RootCommand is created. This command cannot be changed later.

This creates a problem for hosts (see PR #919). If the host has a builder, it instantiates the CommandBuilder, which should then be configured, and finally the command created and added - so that it can use the new configuration.

[KathleenDollard]

Closing this to keep the conversation in #918