Add an explicit distinction between a command's description and summary.
ff4dd9a8e2255f998fd2f017205d9f908f82b8de broke some users who were
relying on only the first line of a command's description being included
in its runner's usage as a summary. That is once again the default
To support the use case described in #42, there's a new Command.summary
getter that explicitly controls the summary included in the parent
runner's usage. This defaults to the first line of the description, but
may be overridden to declare a multi-line summary.