Add tmpl_render(), a function to provide a primitive template
renderer. It takes a dictionary for values to replace variables
shaped {some-variable} in templates found by their name. For now, the
templates are defined in the templates module instead of being read
from a template directory. The values may be lists, in which case
they are rendered with a delimiter, defaulting to ",".
Using an existing template engine like jinja2 is tempting but would
introduce additional dependencies jw-pkg is trying hard to avoid.
Signed-off-by: Jan Lindemann <jan@janware.com>