feat: completions command

[pd93]

Fixes #293

A draft PR based off this comment.

It's a pretty simple implementation. We have a new internal package called completion. This will embed any files in the internal/completion/templates/* directory and when the --completion <shell> flag is given, it will read the template with the corresponding template name, insert any template values and return the string which will then be output to stdout.

I've added a task.tpl.example file to illustrate how you would add a completions script. I'm not super proficient with completion scripts (I have a bit of experience with ZSH), so maybe if we're happy with this approach, we can merge with no templates and slowly migrate the old scripts to this new concept.

I wasn't sure what the best format for the template filenames was. I think we want them to end with .zsh/.bash etc so we get syntax highlighting, but I also added the .tpl prefix so that anyone using a go templating extension/plug could add .tpl.bash etc to the list of templated files without also targeting normal .bash files. Let me know if you think there's a better way?