Add an option to detect node at runtime

Sometimes builds are done once and used in both the browser and in node, so add
an option to do runtime detection if necessary

tests/api.rs

@@ -3,6 +3,7 @@ extern crate test_support;
 #[test]
 fn works() {
     test_support::project()
+        .detect_node(true)
         .file("src/lib.rs", r#"
             #![feature(proc_macro)]