Pass context through operators

Allows more sophisticated functionality

pkg/yqlib/context.go

@@ -0,0 +1,18 @@
+package yqlib
+
+import "container/list"
+
+type Context struct {
+	MatchingNodes *list.List
+	Variables     map[string]*list.List
+}
+
+func (n *Context) SingleChildContext(candidate *CandidateNode) Context {
+	elMap := list.New()
+	elMap.PushBack(candidate)
+	return Context{MatchingNodes: elMap, Variables: n.Variables}
+}
+
+func (n *Context) ChildContext(results *list.List) Context {
+	return Context{MatchingNodes: results, Variables: n.Variables}
+}