hide timestamp if the program is run not in TTY

[smallhive]

Closes #936.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 26.66%. Comparing base (e016f43) to head (ebaaa70).
Report is 21 commits behind head on master.

Files	Patch %	Lines
cmd/s3-gw/app_settings.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
- Coverage   26.71%   26.66%   -0.05%     
==========================================
  Files          87       88       +1     
  Lines       14246    14385     +139     
==========================================
+ Hits         3806     3836      +30     
- Misses      10017    10120     +103     
- Partials      423      429       +6     

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