RFR: 314: Add help command to git-pr

[Erik Helin]

Hi all,

please review this patch that enhances the default `help` command for `git pr`
by quite a bit. `git pr` will now be default list the usage and flags for all
available sub-commands. A user can now also run `git pr help <sub-command>`,
e.g. `git pr help set`:

$ git pr help set
- set
  Usage: git pr set [<ID>]
  Flags:
    -u, --username NAME   Username on host
    -r, --remote NAME     Name of remote, defaults to 'origin'
        --assignees LIST  Comma separated list of assignees
        --title MESSAGE   The title of the pull request
        --open            Set the pull request's state to open
        --closed          Set the pull request's state to closed
        --body            Set the body of the pull request
        --no-draft        Mark the pull request as not draft
        --verbose         Turn on verbose output
        --debug           Turn on debugging output
        --version         Print the version of this tool

Testing:
- Manual testing of `git pr` on Linux x64

Thanks,
Erik

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

Commit messages:
 - skara-314

Changes: https://git.openjdk.java.net/skara/pull/521/files
 Webrev: https://webrevs.openjdk.java.net/skara/521/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-314
  Stats: 896 lines in 18 files changed: 282 ins; 111 del; 503 mod
  Patch: https://git.openjdk.java.net/skara/pull/521.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/521/head:pull/521

PR: https://git.openjdk.java.net/skara/pull/521