Move locale based date behind a flag

[meain]

This makes sure the date behaviour is kept to previous default.
#828

---

TODO

• Use cargo fmt
• Add necessary tests
• Add changelog entry
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[codecov-commenter]

Codecov Report

Merging #840 (60597a7) into master (ff3e48e) will increase coverage by 0.02%.
The diff coverage is 91.66%.

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@@            Coverage Diff             @@
##           master     #840      +/-   ##
==========================================
+ Coverage   87.42%   87.45%   +0.02%     
==========================================
  Files          45       45              
  Lines        4302     4320      +18     
==========================================
+ Hits         3761     3778      +17     
- Misses        541      542       +1     

Impacted Files	Coverage Δ
src/app.rs	39.28% <50.00%> (ø)
src/meta/date.rs	94.28% <93.75%> (-0.25%)	⬇️
src/flags/date.rs	95.77% <100.00%> (+0.18%)	⬆️

... and 1 file with indirect coverage changes

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