[Feature] lerna --dryrun

Hi,

Really liking lerna so far. However I've encountered some unexpected behavior that wound up getting pushed to our public repo and published to npm.

I'd like to be able to run lerna publish to the point of creating the version commit and tag and stop it there so I can inspect what it did before proceeding.

My first thought for an interface for this is:

$ lerna prepublish
... make sure everything's as expected...
$ lerna publish

Where in this case lerna publish would see that the head revision is a version tag and skip the prepublish steps.

It might also be nice to have a lerna revert that's basically just git reset --hard HEAD^ after verifying that the HEAD revision is a version commit.

I'd be happy to do the work for this and submit a PR if you're open to the idea. How does it sound? Ideas for a better interface?

Thanks!

Bo