Configurable command builder

[KathleenDollard]

Fixes #924

As described in issue #924 use of host builders (see #919) is blocked because the Command is added to the CommandLineBuilder prior to configuration.

There are several ways to fix this. I chose this approach because:

• Changing the behavior of RootCommand would be huge and breaking. Not passing anything to this gives you a pleasant default based on your executable and defaults. It's common to build on this.

• I wanted to make it as hard as possible to set the command. In almost all cases, it should be set at builder instantiation as happens today.

• I wanted the configurable builder pattern to be usable beyond the current host to other hosting scenarios (other host interfaces).

I am not happy with the name, but thought it was at least immensely clear.

[KathleenDollard]

Let's look for a way to do this that is lighter than creating a new class.