Default values
**************

For elements such as arguments and options, if "default" property is
set to a string, its value is stored in case the element was not used
(if it's not a string, it'll be used as "init" property instead, see
Argument and option actions).

   parser:option("-o --output", "Output file.", "a.out")
   -- Equivalent:
   parser:option "-o" "--output"
      :description "Output file."
      :default "a.out"

   $ lua script.lua

   {
      output = "a.out"
   }

The existence of a default value is reflected in help message, unless
"show_default" property is set to "false".

   $ lua script.lua --help

   Usage: script.lua [-h] [-o <output>]

   Options:
      -h, --help            Show this help message and exit.
      -o <output>, --output <output>
                            Output file. (default: a.out)

Note that invocation without required arguments is still an error.

   $ lua script.lua -o

   Usage: script.lua [-h] [-o <output>]

   Error: too few arguments


Default mode
============

"defmode" property regulates how argparse should use the default value
of an element.

By default, or if "defmode" contains "u" (for unused), the default
value will be automatically passed to the element if it was not
invoked at all. It will be passed minimal required of times, so that
if the element is allowed to consume no arguments (e.g. using ":args
"?""), the default value is ignored.

If "defmode" contains "a" (for argument), the default value will be
automatically passed to the element if not enough arguments were
passed, or not enough invocations were made.

Consider the difference:

   parser:option "-o"
      :default "a.out"
   parser:option "-p"
      :default "password"
      :defmode "arg"

   $ lua script.lua -h

   Usage: script.lua [-h] [-o <o>] [-p [<p>]]

   Options:
      -h, --help            Show this help message and exit.
      -o <o>                default: a.out
      -p [<p>]              default: password

   $ lua script.lua

   {
      o = "a.out"
   }

   $ lua script.lua -p

   {
      o = "a.out",
      p = "password"
   }

   $ lua script.lua -o

   Usage: script.lua [-h] [-o <o>] [-p [<p>]]

   Error: too few arguments
