Allow user to limit version argument to --version

[kfsone]

Per #102 -- introduces a compiler directive to allow "--version" to be restricted to the long-form only, and a CMakeLists.txt option for controlling it.

Example with it disabled:

set (ARGPARSE_LONG_VERSION_ARG_ONLY ON)
add_subdirectory(argparse)

produces:

> .\build\cparse\cparse.exe --help
Usage: cparse [options]

Optional arguments:
-h --help       shows help message and exits
--version       prints version information and exits
-v --verbose    Increase output verbosity
-q --quiet      Decrease output verbosity
-G --gui        Enable GUI [default: false]

where as without this option on:

> .\build\cparse\cparse.exe --help
Usage: cparse [options]

Optional arguments:
-h --help       shows help message and exits
-v --version     prints version information and exits
-v --verbose    Increase output verbosity
-q --quiet      Decrease output verbosity
-G --gui        Enable GUI [default: false]