Add zsh completion and docs

[kenden]

Implement #1794

This is based on the zsh completion from kops, at kubernetes/kops#2406

[minikube-bot]

Can one of the admins verify this patch?

[codecov-io]

Codecov Report

Merging #2194 into master will decrease coverage by 0.87%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2194      +/-   ##
==========================================
- Coverage   28.62%   27.75%   -0.88%     
==========================================
  Files          82       82              
  Lines        5369     5535     +166     
==========================================
- Hits         1537     1536       -1     
- Misses       3637     3803     +166     
- Partials      195      196       +1

Impacted Files	Coverage Δ
cmd/minikube/cmd/completion.go	1.04% <0%> (-6.96%)	⬇️
pkg/util/kubeconfig/config.go	47.61% <0%> (-0.69%)	⬇️

---

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 10f2bb5...97edb74. Read the comment docs.

[r2d4]

I don't have a zsh to test this against right now, but this overall LGTM and I'm sure some users will really appreciate it.

[r2d4]

Thanks!