Add Zulip as a supported issue source

[RitikaHiremath]

Prerequisites

• I adhere to the coding conventions (described here) in my code.
• I have updated the copyright headers of the files I have modified.
• I have written appropriate commit messages, i.e., I have recorded the goal, the need, the needed changes, and the location of my code modifications for each commit. This includes also, e.g., referencing to relevant issues.
• I have put signed-off tags in all commits.
• I have updated the changelog file NEWS.md appropriately.
• I have checked whether I need to adjust the showcase file showcase.R with respect to my changes.
• The pull request is opened against the branch dev.

Description

Add Zulip as a supported issue source in coronet.

• Extend issues.from.source in util-conf.R to allow zulip as a valid value
• Add reading functionality for Zulip issues in util-read.R
• Update README.md to document zulip as a valid option for issues.from.source

Changelog

Add Zulip as a supported issue source for issues.from.source

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.59%. Comparing base (2ab1212) to head (4285034).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #293      +/-   ##
==========================================
+ Coverage   82.57%   82.59%   +0.01%     
==========================================
  Files          16       16              
  Lines        5378     5377       -1     
==========================================
  Hits         4441     4441              
+ Misses        937      936       -1     

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[bockthom]

Thank you for contributing this enhancement to coronet! Looks good to me. I will merge right away.