RFR: 179: Make git-pr flags configurable

[Erik Helin]

Hi all,

please review this patch that makes all the flags to `git-pr` configurable. It
is also possible to configure different values for flags depending on command,
e.g.:

[pr]
        default = list

[pr "create"]
        assignees = rwestberg

[pr "list"]
        assignees = ehelin

The above is a slightly contrived example just to show how it works. A more
realistic example is my current configuration for `git-pr`:

[pr]
	default = list

[pr "create"]
	ignore-workspace = true
	ignore-local-commits = true
	publish = true
	jcheck = true

Thanks,
Erik

## Testing
- [x] Manual testing of `git-pr` on Linux x64

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

Commits:
 - b599d0c1: skara-179

Changes: https://git.openjdk.java.net/skara/pull/335/files
 Webrev: https://webrevs.openjdk.java.net/skara/335/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-179
  Stats: 122 lines in 1 file changed: 75 ins; 16 del; 31 mod
  Patch: https://git.openjdk.java.net/skara/pull/335.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/335/head:pull/335

PR: https://git.openjdk.java.net/skara/pull/335