Preview next version number at start of release process

I always get nervous using `aegir release` because I'm worried I've not run the right command to release the version I want to release. I don't want to accidentially release a patch for a major change and I don't want to accidentially release a major version for a 0.x project.

It would be cool if the first thing AEgir did when running release was just print the version it's going to release, and pause for a second before continuing - that would give me a chance to ctrl+c if I got it wrong, before `git tag` and `npm publish` is done.