FR: More useful message once exported

[cheshire]

stack-pr prints a somewhat verbose usage string on every export. This is perhaps useful on first couple of exports for users new to tool, but for those who have done more than a couple exports it occupies half the screen and does not contain the required information (e.g. I still have to run git log to see the URLs to linked PRs). Something shorter like just the list of links to PRs could be more useful.

[ZolotukhinM]

Fixed by #108 and #109. There might be more improvements in that area, but at least it's partially addressed now.