hugolib: Implement "related content"

This closes #98, even if this commit does not do full content text search.

We may revisit that problem in the future, but that deserves its own issue.

Fixes #98

tpl/collections/init.go

@@ -63,6 +63,13 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.KeyVals,
+			[]string{"keyVals"},
+			[][2]string{
+				{`{{ keyVals "key" "a" "b" }}`, `key: [a b]`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.In,
 			[]string{"in"},
 			[][2]string{