Support "commenting out" / ignoring nodes easily

[ingydotnet]

It should be possible to ignore any given node (or mapping pair) with a minimal change in the file.

I think a tag is likely the way to go. Something like !X maybe.
The std comment function returns nil which is not the same as ignoring.

Uses I can see right away:

• Ignore a mapping pair.
• Ignore a sequence entry
• Ignore remaining pairs in mapping
• Ignore remaining entries in sequence
• Ignore the next n pairs or entries
• Ignore an entire document in a multi-doc stream

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Idea inspired by yaml/yaml-spec#330

[nielsnl68]

See my reply on my original request mentioned above.