By splitting the namespace test:units on :, we'd find our TestCommand,
which knew nothing of a units method.
So check that a found command also includes the command name we're trying
to call.
Since `Thor::HELP_MAPPINGS` contains `-D`, so `bin/rails -D` show
rails's help. But, in Rails 5.0.1, `bin/rails -D` show the description
of rake task. I think that it is better to have the same behavior.
If we're gonna do this right, it will look mighty different from this anyway.
(Looking at you, Rails 5.1).
It isn't being used in any code as of now, so yanking is the best option.
The API isn't ready yet, which means we haven't documented it.
People can't make their own commands, so there's no reason to show it
in generated documentation.
Reading `Rails::Commands::Command` feels excessive. Especially if users can subclass command
to write their own commands — which I'd like to aim for.
Switch to `Rails::Command` before we get too far into things.
