Allow multiple merge keys (syntax like ::each and ::call)

[scravy]

::each and ::call have their arguments within the key, i.e.

::each >arguments here<: ...

and

::call template:
  arg1: ...
  arg2: ...

The same syntax could be used for ::merge to allow multiple merge keys in an object, i.e. we can have:

::let:
  some:
    foo: one
    bar: two
  more:
    qux: three
    quuz: four
object:
  property: value
  ::merge some: {}
  ::merge more: {}

The actual argument should be ignored (could also be null instead of the empty object {}).

Depends on #10 . #3 would be nice to have for this too.