allow editFile to cascade callbacks.

[mshima]

Allows needles to cascade with file defined by the needle:

insertFooInSpecificFile(generator)(
  insertBar(),
  insertBar2()
)

Old notation needs the file:

generator.editFile(
  'specificFile',
  insertFooInSpecificFile(),
  insertBar(),
  insertBar2()
)

editFile is new so it’s not a breaking change.

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