Add YAML tag for flattening arrays

[robbiemcmichael]

YAML allows objects to merged with <<, but it has no way of flattening arrays. Being able to flatten arrays would allow a subset of the elements to be defined by a Dhall expression.

Proposed syntax:

array: !flatten
  - - 1
    - 2
  - !dhall |
    [3, 4]

This should evaluate to:

array:
  - 1
  - 2
  - 3
  - 4