RFR: contributor command: better validation of input

[Erik Helin]

Hi all,

please review this patch that adds a bit more strict validation of the input to
the `/contributor add` command. This is done to ensure that the resulting
`Co-authored-by` trailer in the final commit message will be approved by jcheck.

Testing:
- Added two additional unit tests for validation
- `make test` passes on Linux x64

Thanks,
Erik

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Commits:
 - df27f322: contributor command: better validation of input

Changes: https://git.openjdk.java.net/skara/pull/421/files
 Webrev: https://webrevs.openjdk.java.net/skara/421/webrev.00
  Stats: 84 lines in 3 files changed: 80 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/421.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/421/head:pull/421

PR: https://git.openjdk.java.net/skara/pull/421