Replace deprecated optparse with argparse, per issue 683 [1]

1: https://github.com/thinkle/gourmet/issues/683
s/options/args/ to make bin/gourmet launch.
Add PYTHON_ARGCOMPLETE_OK string to bin/gourmet.
Replace deprecated optparse with argparse, per issue 683 [1]

1: https://github.com/thinkle/gourmet/issues/683
s/options/args/ to make bin/gourmet launch.
Add PYTHON_ARGCOMPLETE_OK string to bin/gourmet.
Drop Python version 2.6 in INSTALL.md.
Merge branch 'argparse' of https://github.com/thinkle/gourmet into argparse
Merge branch 'argparse'