[WiP] Add some multi-document output support

[gabriel-samfira]

This change adds a new commandlet that returns a helper type which represents a multi-document yaml.

Usage:

PS /> $multiDoc = New-PowershellYamlMultiDocument 
PS /> $multiDoc.Add(@{"hello"="world"})          
PS /> $multiDoc.Add(@{"goodbye"="world"})
PS /> cty $multiDoc
---
hello: world
---
goodbye: world

Or:

PS /> $multiDoc = New-PowershellYamlMultiDocument @(@{"hello"="world"}, @{"goodbye"="world"})
PS /> cty $multiDoc
---
hello: world
---
goodbye: world

Needs tests.

Fixes: #187

CC: @pacorreia