RFR: 443: /reviewer pull request command should accept multiple reviewers

[Erik Helin]

Hi all,

please review this patch that makes the `/reviewer` pull request command accept multiple reviewers at once.

Testing:
- [x] Added a unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - 443

Changes: https://git.openjdk.java.net/skara/pull/687/files
 Webrev: https://webrevs.openjdk.java.net/skara/687/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-443
  Stats: 88 lines in 3 files changed: 61 ins; 5 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/687.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/687/head:pull/687

PR: https://git.openjdk.java.net/skara/pull/687