Support -h help option to add user friendliness (and follow the de-facto standard)

[GHPS]

Follow up to PR#675 with rebased commit and tests.

[GHPS]

Follow up to PR675 with fresh source base and no merge conflicts....

[codecov]

Codecov Report

Merging #700 (7adbadc) into master (185eb4d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #700   +/-   ##
=======================================
  Coverage   75.51%   75.51%           
=======================================
  Files           8        8           
  Lines        1168     1168           
  Branches      295      295           
=======================================
  Hits          882      882           
  Misses        205      205           
  Partials       81       81           

Impacted Files	Coverage Δ
tmuxp/cli.py	73.23% <100.00%> (ø)

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[tony]

@GHPS Thank you, Well done!