Worked on tools to make 'tagging' easier.

It turns out that a common pattern is to have several integrating projects.
When releasing/bundling it is necessary to create a 'snapshot' of all
submodules.

We should now do this in the following way:
1. Create a branch for the new release
2. Run 'substance --tag'. This changes the project.json (adding absolute shas),
   and a npm-shrinkwrap.json (which makes npm install checkout the very same configuration)
3. Commit the changes to the release branch.