Add package version file

[kateyy]

Add ability to request minimum version of cmake-init base-projects in find_package.
The CMake documentation suggests to generate and deploy a Package Version File. Besides template-config.cmake, projects would than also deploy template-config-version.cmake and allow to be included via calls like find_package(template_foo 2.0 REQUIRED).

As far as I can see, there is currently no way for users to check in CMake for versions of cmake-init-based projects, which is quite unfortunate.