Yq: Provide a way to persist document delimiters (`---`) when reading a multi-document file

[bioball]

Currently, reading multiple documents strips the --- delimiter that denotes the start of each document. For instance, take the following file:

---
a: 1
---
a: 2
---
a: 3

And run the following

yq r -d'*' test.yaml ""

It outputs the following:

a: 1
a: 2
a: 3

The output here is invalid yaml, and moreover; there might be times where you might want to keep the output as multiple documents. Ideally there should be a way to produce this output:

---
a: 1
---
a: 2
---
a: 3

[mikefarah]

Going to add a way of doing this in v4 (currently in dev)