Add support for jinja templating to inject metadata into e.g. documentation or easily generate other formats

[apirogov]

We were thinking about allcontributors support anyway.

Furthermore, somesy should be able to generate something like the AUTHORS.md embedded in docs.

If we add a jinja backend with template files, both kind of use-cases could be covered easily (also see #40 and #29 )

This would just require to build a custom jinja context with access to a loaded config and expose a new command to generate a file from a template that expands in the project metadata.