Add template version to template

**Is your feature request related to a problem? Please describe.**
We'd like to be able to track the version numbers of the templates.

**Describe the solution you'd like**
We can add a _template_version in the copier yaml file.

**Describe alternatives you've considered**
The recommended approach in the copier docs is to use github submodules.

**Additional context**
Add any other context or screenshots about the feature request here.