feat(mako): generic source/output mappings

This includes proper handling of dependencies.
The code is concise, pythonic and quite 'cody', but does the job just
fine.

etc/api/shared.yaml

@@ -13,6 +13,10 @@ api:
     - name: youtube
       version: v3
   base_path: "etc/api"
+  templates:
+  # all output directories are relative to the one set for the respective API
+    - source: cargo.toml
+      # output_dir:  optional - not there if unset
 cargo:
   build_version: "0.0.1"
   authors: