Track indentation in yaml.Node

[rbren]

I'm really enjoying the ability to use yaml.Node to modify a YAML document while mostly preserving comments and formatting. The one big gap is the inability to preserve indentation.

I've found a couple related issues, but they don't quite get at the problem:

• v3 - Configurable or autodetected list indentation #720 (specifically about list indentation)
• v3: preserve comments lines and indents after unmarshal -> marshal #709 (mainly about a bug in preserving comments)

I would be happy to take a stab at implementing this if y'all would accept a PR (and maybe provide some guidance).

[Justinzobel]

Yes the inability to preserve existing indentation is certainly a problem.