RFR: 211: git-pr: add status subcommand

[Erik Helin]

Hi all,

please review this patch that adds the beginnings of the `git pr status`
subcommand. The output of `git pr status` is currently:

$ git pr status 339
Status: RFR
Checks:
- jcheck: OK

I plan to add more information for individual checks. There are also two flags
supported, `--no-decoration` and `--no-checks`. If a user just wants the "raw"
status on standard out, then the following would be used:

$ git pr status --no-decoration --no-checks 339
RFR

This output is mainly intended for those that for example want to use the status
of a pull request in their own scripts.

Thanks,
Erik

## Testing
- [x] Manual testing of `git pr status`

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

Commits:
 - 72f4c775: skara-211

Changes: https://git.openjdk.java.net/skara/pull/340/files
 Webrev: https://webrevs.openjdk.java.net/skara/340/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-211
  Stats: 134 lines in 1 file changed: 109 ins; 20 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/340.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/340/head:pull/340

PR: https://git.openjdk.java.net/skara/pull/340