RFR: git-pr: add contributor sub-command [v2]

[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

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Fix helptext
  
  Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/701/files
  - new: https://git.openjdk.java.net/skara/pull/701/files/84ed8656..f0bcbe25

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/701/webrev.01
 - incr: https://webrevs.openjdk.java.net/skara/701/webrev.00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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