Add bash completion

[balki]

POC for bash completion support. It does two things as for now

1. Following then, it completes a verb
2. When tab is hit twice after completing a verb, it shows help for the verb

Demo

Setup

❯ mlr completion     
Usage: mlr completion SHELL
Supported shells: bash

Add below to your bashrc to enable completion
source <(mlr completion bash)

❯ mlr completion bash
complete -o nospace -o nosort -C "mlr _complete_bash" mlr

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Let me know if you like the approach. Can be enhanced further

• zsh completion
• complete global flags
• complete verb flags
• complete field names from input for some verbs like cut -f.. (extracted from first line of input in --from flag)

[balki]

@johnkerl Are you ok with the approach for completion? Reading flags may require bit of refactor in command line parsing logic.

[johnkerl]

@balki Hi and sorry for the delay!

This is AWESOME stuff!!! 😎

Agreed re all four checkboxes -- if you're up for it, these are all great! It's better to have some completion than none at all, so, I'm happy to take these as separate PRs.

I think bash is more popular than zsh but I really don't have data -- in any case ideally one would have support for both.

I'm happy to work with you on refactoring the command-line-parsing logic ...

[johnkerl]

Please see requested change

[balki]

Thank you! Will try to work on this in next few weeks.

Yes bash is more popular and will start with bash as it is simpler. However zsh completion is more powerful and useful. E.g the flag completion has the help text which is very helpful. And mac os default shell is zsh.