Use real types to parse labels

We were using `yaml.Unmarshal` on basic types or inline structs. This
was not efficient and not clear to defined what we expect in labels.
We now use types to unmarshal the labels.

Only the `values` label is, at this time, parsed by GetValuesFromLabel
because this `utils` function is clearly a special case.

doc/mkdocs.yml

@@ -49,8 +49,10 @@ nav:
   - FAQ: faq.md
   - Go Packages:
     - packages/cmd/katenary.md
-    - packages/generator.md
     - packages/parser.md
     - packages/update.md
     - packages/utils.md
-    - packages/generator/extrafiles.md
+    - Generator: 
+      - Index: packages/generator.md
+      - ExtraFiles: packages/generator/extrafiles.md
+      - LabelStructs: packages/generator/labelStructs.md