R4R: make gaiacli config handle "indent" flag

[alessio]

It defaults to false, users can override it via gaiacli config
and avoid to append --indent to all commands:

$ gaiacli config indent true

Closes: #2607

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[jackzampolin]

Good addition! Thanks @alessio

[codecov]

Codecov Report

Merging #3342 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #3342      +/-   ##
==========================================
+ Coverage     55.6%   55.6%   +<.01%     
==========================================
  Files          132     132              
  Lines         9559    9556       -3     
==========================================
- Hits          5315    5314       -1     
+ Misses        3907    3905       -2     
  Partials       337     337

[cwgoes]

ACK