Make output of krew search easier to read

[corneliusweig]

So far, the last column was labelled with STATUS and had three options
(installed, available, unavailable). This was hard to read because
installed and available have the same length, and unavailable was not
specific.

Now, the column is labelled INSTALLED with values
yes/no/unavailable on <PLATFORM>

In addition, I increased the padding between columns to two spaces which looks cleaner. WDYT?

---

Sample output:

NAME                            DESCRIPTION                                         INSTALLED
access-matrix                   Show an access matrix for server resources          no
bulk-action                     Do bulk actions on Kubernetes resources.            unavailable on linux
ca-cert                         Print the PEM CA certificate of the current clu...  yes

Fix #250

[codecov-io]

Codecov Report

Merging #253 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #253   +/-   ##
======================================
  Coverage    53.6%   53.6%           
======================================
  Files          16      16           
  Lines         735     735           
======================================
  Hits          394     394           
  Misses        289     289           
  Partials       52      52

---

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[ahmetb]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, corneliusweig

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