Add convert rules.

rules/rules_test.go

@@ -46,6 +46,13 @@ func TestConverts(t *testing.T) {
 			"f",
 			71.6,
 		},
+		{
+			"k to f",
+			22.0,
+			"k",
+			"f",
+			-420.07,
+		},
 	}
 
 	ngn, err := New()