Support a configuration file

[brettcannon]

Exists for the Windows launcher: https://www.python.org/dev/peps/pep-0397/#configuration-file.

A key thing to remember is should not get to the point that you're using this to alias specific interpreters, just making it easier to specify constraints on what kind of interpreter you need and then letting the launcher pick for you. Otherwise you should just launch the specific interpreter you want.

• Customized commands?
  • Want a better format like TOML?
  • Want a way to override/specify things, e.g. wanting a framework build on macOS?
    • Aliasing? E.g. 2.7-framework for
      /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python?
    • Just provide a way to specify a specific interpreter for a specific version?
      E.g. 2.7 for
      /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python
    • What about implementations that don't install to e.g. python3.7 like pypy3?
      • Need more than just being able to alias pypy3 to its Python version?
  • How should the config file search work?
    • Pre-defined locations?
    • Walk up from current directory?
    • XDG base directory specification?