Use a proper tag for !!merge << keys

[eemeli]

Fixes #579

Let's handle !!merge << like other explicit YAML 1.1 tags, so that it works even if not separately enabled, and fix the options so that merge: true is a shorthand for customTags: ['merge'] (and combines with that option nicely, if they're both used).

This turned out to be a bit tricky to do "right", while also not introducing any breaking changes. In YAML 1.1, << is special, but only if it's used as a bare mapping key, and so I added an atKey flag into the ComposeContext and a default: 'key' value for tags, so that composition can take the node position into account.

The difficulty here is slightly compounded by a mistake we made early on (#22, CC @ikatyang) in mis-reading the !!merge spec and allowing multiple << keys to be present in a single mapping, which is not valid YAML -- but removing this support now would count as a breaking change, and that would be unfortunate.

While at this, I took the opportunity to add an optional addToJSMap(ctx, map, value) method that !!merge can use to customize how it's added to a JS map, so that its handling is now much less custom.