Add GELF support

[lingfish]

Implement the ability to log to a GELF server/input, via the use of pygelf.

This is my initial stab at it; similar to SyslogClient, I opted to use parsed_aggregate_reports_to_csv_rows.

This is all sent in one field to GELF (parsedmarc), where then it is normal practice to split the sub-fields out within your logging app (eg. Graylog).

This would be an example Graylog pipeline rule:

rule "DMARC GELF"
when
  has_field("parsedmarc")
then
  let json = parse_json(to_string($message.parsedmarc));
  let map = to_map(json);
  set_fields(map, "dmarc_");
end

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.98%. Comparing base (d6128ea) to head (fdfb92b).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   59.88%   59.98%   +0.10%     
==========================================
  Files          12       12              
  Lines        1578     1577       -1     
==========================================
+ Hits          945      946       +1     
+ Misses        633      631       -2     

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[seanthegeek]

Thanks. Can you add documentation updates to this PR?

[lingfish]

Thanks. Can you add documentation updates to this PR?

Err, forgive me... I thought I had?

[lingfish]

Hi @seanthegeek just wondering if we're all good here?