Allow multi recipients for email notifier

[crazy-max]

Fixes #450

cc @romualdvilletet

[codecov]

Codecov Report

Merging #463 (4b7441f) into master (bbb00c0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #463   +/-   ##
=======================================
  Coverage   70.80%   70.80%           
=======================================
  Files          10       10           
  Lines         435      435           
=======================================
  Hits          308      308           
  Misses         88       88           
  Partials       39       39           

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[romuald-villetet]

@crazy-max Thanks for this!