Works, need better doc & so on, but a bit of user feedback will help users

ref: https://github.com/varnish/varnish-modules/issues/137

Old install_${PLATFORM} lingers as they are references from previous
releases.

Remove repo.v-c.o reference.

Closes #72

Ref #72

This adds a "branch" attribute to the rev objects.

If an "fmt" object exists with attributes "url_vcc" and/or "url_doc",
those are taken as printf formats (which should contain a single %s) and
used with the branch attributes from the revs.