Feat/preserve maildir subdirectories

[SvenRohrTNG]

Resolves #156

Hi @michaeladler

please triple check, since I am new to rust ;-)
Has been running successfully on my local machine for the last hour or so :-)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.40%. Comparing base (8a5b8b0) to head (43e1366).

Files with missing lines	Patch %	Lines
src/lib/action.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #162   +/-   ##
=======================================
  Coverage   62.40%   62.40%           
=======================================
  Files           6        6           
  Lines         508      508           
=======================================
  Hits          317      317           
  Misses        191      191           

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[michaeladler]

Looks good. Just fix the formatting (rustfmt) and we can merge this :)