feat: add --show-item-count (-C) option, resolves #331

[codecov[bot]]

Codecov Report

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

Project coverage is 90.19%. Comparing base (186a605) to head (0f371f6).

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #332 +/- ## ========================================== - Coverage 90.30% 90.19% -0.11% ========================================== Files 32 32 Lines 3465 3469 +4 ========================================== Hits 3129 3129 - Misses 272 275 +3 - Partials 64 65 +1 ```

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[dundee]

Thanks for your contribution!

Resolves #331.