Add an explicit distinction between a command's description and summary.

CHANGELOG.md

+## 0.13.4
+
+* By default, only the first line of a command's description is included in its
+  parent runner's usage string. This returns to the default behavior from
+  before 0.13.3+1.
+
+* A `Command.summary` getter has been added to explicitly control the summary
+  that appears in the parent runner's usage string. This getter defaults to the
+  first line of the description, but can be overridden if the user wants a
+  multi-line summary.
+
 ## 0.13.3+6
 
 * README fixes.
 
 ## 0.13.3+5
 
 * Make strong mode clean.
 
 ## 0.13.3+4
 
@@ skipping 95 matching lines @@
 
 ## 0.11.0
 
 * Move handling trailing options from `ArgParser.parse()` into `ArgParser`
   itself. This lets subcommands have different behavior for how they handle
   trailing options.
 
 ## 0.10.0+2
 
 * Usage ignores hidden options when determining column widths.