ENH: add nbqa-shell

[pwoolvett]

Enable a new flag --nbqa-shell. When set, instructs nbqa to run the command argument directly, instead of importing it as a module via python.

• updated docs in examples to include reference usage.
• updated cli usage message to include new flag.
• add new exception used when the requested script is not found.
• update interal api when shell is required:
  • main._run_command
  • main._check_command_is_installed
  • cmdline.CLIArgs
  • config.config.Configs
  • config.config.get_default_config
• added tests for cli flag:
  • Fallback to default usage when flag not specified.
  • Use sp.run directly instead of python -m usage when flag is set.
  • raise exception when flag is passed and command is not found.

fixes #695

[MarcoGorelli]

Hi @pwoolvett - reckon this option needs to be configurable from pyproject.toml?

[MarcoGorelli]

Fixed this up a bit so it's configurable from pyproject.toml - thanks for your PR!