Preserve comments in YAML/TOML in the Remarshal func

[bep]

Do some string replacement

"# My comment" => "hugocomment = "My comment" => and back.

Ignore the last step for JSON (which does not support it).

[kaushalmodi]

For JSON, may be use a hack like shown here: https://stackoverflow.com/a/244858/1219634

..
"_hugo_comment": "My comment",
..