Bash/ZSH auto-complete docs

[masonj5n]

Closes #863

Here's a first pass of updating the docs. Hopefully I cleared up the process of enabling auto-completion. I added some examples of what the auto-complete looks like, they're links to giphy.com, not sure if it'd be better to store the gifs themselves instead of having links. If they don't look like they belong we can definitely remove them. Let me know though and I'll make changes!

[coilysiren]

Thanks for helping out @masonj88 ✨ it's nearly there 👍

[codecov]

Codecov Report

Merging #905 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #905   +/-   ##
=======================================
  Coverage   71.38%   71.38%           
=======================================
  Files          30       30           
  Lines        2394     2394           
=======================================
  Hits         1709     1709           
  Misses        578      578           
  Partials      107      107

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a221e66...e36e86a. Read the comment docs.

[saschagrunert]

LGTM, thanks! :)

[coilysiren]

Oh, this has merge conflicts now because the documentation has moved!

You'll want to make these same changes against the v2 manual file now https://github.com/urfave/cli/blob/master/docs/v2/manual.md

@masonj88

[masonj5n]

I'll have it done soon!

[daxmc99]

Any update on this? Would it be ok if I made another PR to resolve these merge conflicts? This doc is sorely needed

[coilysiren]

@daxmc99 please do!

[coilysiren]

closing this as stale

[coilysiren]

@masonj188 feel free to race @daxmc99 for this PR 😆 but otherwise you can ignore this

[masonj5n]

Really sorry for taking so long, just opened a new pull request for the merge into the new master!