Add bash/zsh auto completion generation

[integrii]

We should generate bash auto complete code when some pre-defined flag is specified. The flag should be able to be turned off if the user prefers that flag not be used by the system.

[philippgille]

Maybe rather a subcommand? Like './myApp installAutoCompletion'. Or make it configurable (app developer can choose between flag and subcommand, as well as change the flag and subcommand).

[integrii]

I think a subcommand makes more sense for sure.

The main challenge I see here is not using a 3rd party package. I want to keep flaggy a zero dependency package.

[WhyNotHugo]

A possible approach to avoiding third party dependencies is implementing this feature in a separate package. I'm thinking of something like this:

• All the command definitions (e.g.: in application code, not here) need to be moved out of the main package.
• The third-party lib can read all of flaggy's commands and settings.
• The third party lib generates the completions.

Note that the approach also allows having a separate binary that generates the completions, so the new library doesn't even need to be build into the application binary itself.