Add prefer-stable flag to pick stable package over unstable ones when possible

You can try the feature using: http://seld.be/composer-preferstable.phar

Add `"prefer-stable":true` in a composer file that has `"minimum-stability"` set to something else than stable, and see what happens with `composer update --dry-run`. It should update packages to stable versions when it can.

Feedback welcome both on the concept and whether the code works at all :)

Happy easter :rabbit: