add: support for sending multiple emails

[Miksus]

Now it should be possible to open the connection and send multiple emails per email session.

NOTE: If someone has subclassed the EmailSender and overridden the connect method, this may break.
The possibility of this is very small though as the method is not well documented.

[codecov-commenter]

Codecov Report

Merging #28 (9e1c0f3) into master (a0d6e85) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   99.59%   99.60%   +0.01%     
==========================================
  Files          12       12              
  Lines         494      508      +14     
==========================================
+ Hits          492      506      +14     
  Misses          2        2              

Impacted Files	Coverage Δ
redmail/email/sender.py	99.35% <100.00%> (+0.06%)	⬆️

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