Make previous version available to templates

[DonalChilde]

Having the previous version number available to templates would allow for easy github compare urls in entry_title_templates.

e.g.

{% if version %}[{{ version }}](https://github.com/Account/Project/compare/{{previous_version}}...{{version}}) — {% endif %} {{ date.strftime('%Y-%m-%d') }}
{{"\n"}}
### Whats Changed in {{version}}