Add ActiveHelp to the cobra-cli

[marckhouzam]

This PR adds ActiveHelp to the cobra-cli as an example of the new ActiveHelp feature.

This is a revival of #24 which I was not able to re-open. Since the original PR was done from the VilledeMontreal organization, I wanted to finish the work there to give proper credit.

To test:

1. checkout the PR branch
2. make (the go.mod in the PR points to the ActiveHelp commit in Cobra)
3. source the updated shell completion script For zsh on Mac:

source <(bin/cobra-cli_darwin_amd64 completion zsh)
compdef _cobra-cli cobra-cli
compdef _cobra-cli cobra-cli_darwin_amd64

For bash on Mac:

source <(bin/cobra-cli_darwin_amd64 completion bash)
complete -o default -F __start_cobra-cli cobra-cli_darwin_amd64

Then try out the ActiveHelp triggers:

$ bin/cobra-cli_darwin_amd64 init <TAB>
Optionally specify the path of the go module to initialize
--
CONDUCT.md       LICENSE.txt      Makefile         bin/             go.mod           main.go
CONTRIBUTING.md  MAINTAINERS      README.md        cmd/             go.sum           tpl/

$ bin/cobra-cli_darwin_amd64 init newprogram <TAB>
This command does not take any more arguments (but may accept flags)

$ bin/cobra-cli_darwin_amd64 init newprogram extra <TAB>
ERROR: Too many arguments specified

$ bin/cobra-cli_darwin_amd64 add <TAB>
Please specify the name for the new command

$ bin/cobra-cli_darwin_amd64 add command <TAB>
This command does not take any more arguments (but may accept flags)

$ bin/cobra-cli_darwin_amd64 add command command2 <TAB>
ERROR: Too many arguments specified

You can compare the new behaviour with the old one by running the same commands but using the main branch or the cobra-cli installed on your workstation, e.g.,

$ cobra-cli add <TAB>
CONDUCT.md       LICENSE.txt      Makefile         bin/             go.mod           main.go
CONTRIBUTING.md  MAINTAINERS      README.md        cmd/             go.sum           tpl/