Helpers for recipe authors

- Add helper function `package-build-create-recipe`
- Add `package-build-minor-mode`, enabled for recipes: C-c C-c builds current recipe
- Opening recipe files causes `package-build` to get loaded, and
  `package-build-minor-mode` to get enabled

Does this look reasonable? I'm aiming to refine the workflow of creating new recipes. Idea is that we can post instructions saying something like:

1. clone the melpa repo
2. load package-build.el
3. `M-x package-build-create-recipe`
4. `C-c C-c` to build the recipe when done
5. Test-install freshly-built package (TBD)