Add help_vertical_space Parser property

Sets number of extra newlines to insert between help strings
for different elements within a help group. Inherited by commands.

CHANGELOG.md

@@ -13,6 +13,9 @@
 * Added `:group(name, ...)` method to Parser and Command objects, allowing
   custom grouping of arguments, options, and commands in autogenerated
   help string.
+* Added `help_vertical_space` property for configuring number of extra
+  newlines between help strings for different arguments and options in
+  autogenerated help string.
 
 ### Improvements