shtab v1.3.0 alpha

• 🔨 fix bash missing custom complete functionality (add_argument().complete = ...)
• 🔨 fix subcommand completion & update examples/customcomplete.py
• 🎉 add shtab.add_argument_to(parser, option_string) (#18)
  • option: adds your_cli_app [--print-completion={bash,zsh}]
  • subcommand: adds your_cli_app completion [{bash,zsh}]
  • handles the print(shtab.complete(parser, shell)) & exit(0) logic too
• 🎉 expose SUPPORTED_SHELLS
• 📖 add & update documentation
• 🔍 add tests
• 🧹 misc tidy