Bash completion

[ghost]

Adds bash completion function. Example:

args::ArgumentParser parser("parser");

// ...

args::CompletionFlag complete(parser, {"complete"});

try
{
    parser.ParseCLI(args, args);
    // ....
}
catch (args::Completion &e)
{
    std::cout << e.what();
    return 0;
}

Under the hood, bash script calls program --complete bash NUMBER_OF_CURRENT_ARGUMENT ARGS. I wanted to avoid rewriting args in bash.

To test this you can build examples with cmake and add bash completion for examples/completion.cxx:

. examples/bash_completion.sh

I broke HelpChoices API (std::string -> std::vector<std::string>). It wasn't released so I guess it's ok. Choices strings are also used by completion.

In theory adding zsh and fish should be relatively easy. I added first argument to --complete which could be used for shell specific code.

[coveralls]

Coverage decreased (-1.6%) to 93.353% when pulling 32a103d on pavel-belikov:bash-completion into c5a6098 on Taywee:master.

[coveralls]

Coverage decreased (-0.1%) to 94.818% when pulling 016304b on pavel-belikov:bash-completion into c5a6098 on Taywee:master.

[coveralls]

Coverage increased (+0.06%) to 95.01% when pulling b8fc68a on pavel-belikov:bash-completion into c5a6098 on Taywee:master.

[Taywee]

Yeah, I'm happy with breaking backwards compatibility between minor releases. If it's warranted enough, I'm happy breaking backwards compatibility after a realease, with major version bump.