Contained overlay directives

config1.yml

---
foo: 42

config2.yml

---
foo: 42
bar:
  ree: deadbeef
  quuz: prefix
list:
- zeroth

overlay.yml

#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.all, expects="1+"
---
#@overlay/match missing_ok=True
bar:
  #@overlay/match missing_ok=True
  #@overlay/remove
  qux: toasty
  #@overlay/match missing_ok=True
  baz: added from overlay
  #@overlay/match when=1
  #@overlay/replace via=lambda left,right: left + "-" + right
  quuz: suffix

#@overlay/match missing_ok=True
list:
- first
#@overlay/insert after=True
- second