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

crates/wasm-bindgen-cli/src/bin/wasm-bindgen.rs

@@ -18,17 +18,19 @@ Usage:
     wasm-bindgen -V | --version
 
 Options:
-    -h --help               Show this screen.
-    --out-dir DIR           Output directory
-    --nodejs                Generate output for node.js, not the browser
-    --typescript            Output a TypeScript definition file
-    --debug                 Include otherwise-extraneous debug checks in output
-    -V --version            Print the version number of wasm-bindgen
+    -h --help                Show this screen.
+    --out-dir DIR            Output directory
+    --nodejs                 Generate output for node.js, not the browser
+    --nodejs-runtime-detect  Detect at runtime whether we're in node or a browser
+    --typescript             Output a TypeScript definition file
+    --debug                  Include otherwise-extraneous debug checks in output
+    -V --version             Print the version number of wasm-bindgen
 ";
 
 #[derive(Debug, Deserialize)]
 struct Args {
     flag_nodejs: bool,
+    flag_nodejs_runtime_detect: bool,
     flag_typescript: bool,
     flag_out_dir: Option<PathBuf>,
     flag_debug: bool,
@@ -54,6 +56,7 @@ fn main() {
     let mut b = Bindgen::new();
     b.input_path(&input)
      .nodejs(args.flag_nodejs)
+     .nodejs_runtime_detect(args.flag_nodejs_runtime_detect)
      .debug(args.flag_debug)
      .typescript(args.flag_typescript);