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
Add blank-mode recipe
Merge pull request #609 from yasuyk/blank-mode

Add blank-mode recipe
Extract new pb/update-or-insert-version function
Ensure recipes.json is rebuilt alongside archive.json (see #610)
HTML index: don't list built packages without recipes (see #610)
HTML index: remove leading whitespace
Added interval-tree
Added clippy
Merge pull request #611 from Fuco1/clippy

Added clippy
Merge pull request #612 from Fuco1/interval-tree

Added interval-tree
Add thread-dump.el
Tidier formulation of archive.json and recipes.json intersection
clarify the README a bit
Add nose (closes #613)
Merge pull request #607 from milkypostman/create-recipe-helpers

Helpers for recipe authors
Add Stephen Bach's port of the VIM lusty buffer explorer.
When writing single-file packages, update Version header

Closes #588
After interactively building a package, prompt to install locally
Add tumble