RFR: git-pr: add contributor sub-command

[Erik Helin]

Hi all,

please review this patch that adds the `git pr contributor` sub-command that corresponds to the
[`/contributor`](https://wiki.openjdk.java.net/display/SKARA/Pull+Request+Commands#PullRequestCommands-/contributor)
pull request command. Contributors who prefer the command line can now add contributors to a pull request, for example
`git pr contributor --add='@edvbld'` or `git pr contributor --add=ehelin`.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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

Commit messages:
 - git-pr: add contributor sub-command

Changes: https://git.openjdk.java.net/skara/pull/701/files
 Webrev: https://webrevs.openjdk.java.net/skara/701/webrev.00
  Stats: 93 lines in 2 files changed: 92 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/701.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/701/head:pull/701

PR: https://git.openjdk.java.net/skara/pull/701