RFR: git-pr: add reviewer sub-command

[Erik Helin]

Hi all,

please review this patch that adds the `git pr reviewer` sub-command which corresponds to the
[`/reviewer`](https://wiki.openjdk.java.net/display/SKARA/Pull+Request+Commands#PullRequestCommands-/reviewer) pull
request command. A pull request author can now run `git pr reviewr --add=ehelin` to mark me as a reviewer of a pull
request (note that not all OpenJDK projects allow authors to add reviewers).

Testing:
- Manual testing on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - git-pr: add reviewer subcommand

Changes: https://git.openjdk.java.net/skara/pull/697/files
 Webrev: https://webrevs.openjdk.java.net/skara/697/webrev.00
  Stats: 93 lines in 2 files changed: 92 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/697.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/697/head:pull/697

PR: https://git.openjdk.java.net/skara/pull/697