Encode strings with YAML 1.1 boolean values with quotes

[dsharp-pivotal]

To allow better compatibility with YAML 1.1 parsers, when outputting a
string that is equal to one of the YAML 1.1 boolean values ("yes", "no",
"on", "off", etc), enclose the string in quotes. The string will then
parse correctly by either YAML 1.1 or YAML 1.2 parsers.

This change does not affect decoding: Bare yes and no will still be
decoded as strings.

See: #214

[laverya]

Is this functionally different than #490?

(I don't care which fix gets merged. I just want a fix to get merged)

[dsharp-pivotal]

@laverya Oh, I didn't see your PR.

They look pretty similar, but I see one functional difference: This PR also forces quotes when a node is explicitly tagged as !!str (the diff in encoder.node()). My desire for that specifically is described in mikefarah/yq#341

[laverya]

They both make the same (functional) change to encoder.node though?

		canUsePlain = rtag == strTag && !isBase60Float(s) && !isYaml11Bool(s)

vs

		canUsePlain = rtag == strTag && !isBase60Float(s)

		// Check to see if this is a v1.1 yaml bool string
		// if it is, require quotes to preserve backwards compatibility
		_, is11BoolString := boolMap[s]
		canUsePlain = !is11BoolString && canUsePlain

Not that it matters - as I said before, as long as one gets merged I'm happy.

[dsharp-pivotal]

@laverya Yeah, in terms of those changes, looks equivalent to me.

@niemeyer Can you take a look?