Evalsubst operator

[kknicker]

Add an "evalsubst" operator similar to "envsubst", to evaluate values as yq expressions.

It would be nice to have the ability to handle plain expressions or strings with interpolated expressions, and results which can be a string or an object. Examples are below.

yq '(.. | select(tag == "!!str")) |= evalsubst' t.yaml

Example 1:

vars:
  x: 1
  y: 2
z: \(.vars | .x + .y)

vars:
  x: 1
  y: 2
z: 3

Example 2:

vars:
  x: 1
  y: 2
z: The answer is \(.vars | .x + .y)

vars:
  x: 1
  y: 2
z: The answer is 3

Example 3:

vars:
  x: 1
  y: 2
z: \(.vars)

vars:
  x: 1
  y: 2
z:
  x: 1
  y: 2