Add bash completion for docker config command family

[albers]

This adds bash completion for #45.
Ping @sdurrheimer for zsh completion

Additional commits add related functionality/refactorings:

• bash completion for secret inspect --pretty (Ref: Add --pretty option to "secret inspect" and "config inspect" #88)
• a minor refactoring in bash completion for secret rm|remove

[codecov-io]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is n/a.

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##           master    #283   +/-   ##
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  Coverage    48.7%   48.7%           
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  Files         185     185           
  Lines       12168   12168           
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  Hits         5926    5926           
  Misses       5875    5875           
  Partials      367     367

[mlaventure]

@albers do you think it'd be possible to have path completion enabled for config create ?

i.e.:

$ docker config my-<tab>

correctly doesn't offer suggestion

but

$ docker config my-config /etc/<tab>

Doesn't either, and I would have expected it to do so.

[albers]

@mlaventure Nice catch, thanks a lot. I added path completion to docker config create.

[mlaventure]

Thanks!

It works perfectly now, amazing job as always, really appreciated :)

LGTM

[vdemeester]

LGTM 🦁