Add fish completions, update documentation accordingly

[nucleogenesis]

Closes #159

The fish shell's completions docs expect a space-separated list of completions. Piping dtask _completions into the fish utility string-collect appears to do exactly what is needed to setup the completions.

• Adds a file "completions/completions.fish"

Testing

• Install fish
• From your bash shell, run fish to enter a fish shell
• Checkout my branch and run make dist/dstask
• Run dist/dstask fish-completion | source
• Start typing dist/dstask show- and see now that the completions work in Fish

Reviewer guidance

Have I missed anything? I basically just grepped for "bash" and then copy-pasted the seemingly relevant lines changed "bash" to "fish".

I've also updated the documentation to show multiple examples -- one each for bash, zsh and fish. I felt this would be particularly helpful for fish users as the <() operation doesn't work in fish:

fish: Invalid redirection target:
source <(dist/dstask _completions)

Happy to tweak it as necessary.

[naggie]

Thanks @nucleogenesis ! I have bypassed CI because drone appears to have grinded to a halt. The service, and the company, due to the acquisition no doubt. It works locally.

I'll have to get round to moving to github actions soon, and perhaps not vendor modules.