[Sling-framework] Better npm run publish-packages

[caiomdias]

I'm submitting a:

[ ] bug report [x] feature request [ ] support request

Current behavior:

When running npm run publish-packages, the script tries to publish all packages one by one and, if a package wasn't updated, publishing fails and then process goes on to the next package.

Expected behavior:

Instead, Lerna should first identify which packages have changed and publish only these. It works with lerna publish but not with lerna exec, that we have to use.

A possible solution would be creating a bash script with lerna updated and use this information to run npm publish on each changed package.