feat: add --show-mtime (-M) option

[dundee]

fixes #242

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 89.72%. Comparing base (f8802e6) to head (07e78f3).

Files	Patch %	Lines
cmd/gdu/app/app.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
- Coverage   90.05%   89.72%   -0.33%     
==========================================
  Files          36       36              
  Lines        3207     3213       +6     
==========================================
- Hits         2888     2883       -5     
- Misses        247      255       +8     
- Partials       72       75       +3     

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