Support implicit TLS connections

[kewisch]

This PR add support for implicit TLS connections which some servers (e.g. Stalwart) offer. I would normally have renamed the starttls parameter to a more general tls parameter that takes explicit/implicit/None, but this wouldn't be backwards compatible without messy migration code. If you prefer just changing the API and bumping the major version let me know.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.54%. Comparing base (cb690f5) to head (4339726).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
sievelib/managesieve.py	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   87.62%   87.54%   -0.08%     
==========================================
  Files          10       10              
  Lines        2069     2072       +3     
==========================================
+ Hits         1813     1814       +1     
- Misses        256      258       +2     

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[tonioo]

@kewisch Thanks for this contribution

[tonioo]

@kewisch If you still have your dev environment running, could you please check this PR:
#124