Default to ~/.config/gmailctl for newly created config dirs.

[lgarron]

This is a minimal fix for #144

It is a revision of #393 and uses a library for the internal XDG path calculation per feedback there.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.56%. Comparing base (1316284) to head (05bc582).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/root_cmd.go	0.00%	6 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@ Coverage Diff @@## master #455 +/- ##
==========================================
- Coverage 37.60% 37.56% -0.05% 
==========================================
Files 54 54 Lines 3670 3674 +4 ==========================================
Hits 1380 1380 - Misses 2199 2203 +4 
Partials 91 91 

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