Add an example of wasm2asm and wasm-bindgen

This commit adds an example of executing the `wasm2asm` tool to generate asm.js
output instead of WebAssembly. This is often useful when supporting older
browsers, such as IE 11, that doesn't have native support for WebAssembly.

crates/cli/src/bin/wasm2es6js.rs

@@ -27,6 +27,7 @@ Options:
     --typescript            Output a `*.d.ts` file next to the JS output
     --base64                Inline the wasm module using base64 encoding
     --fetch PATH            Load module by passing the PATH argument to `fetch()`
+    --wasm2asm              Convert wasm to asm.js and don't use `WebAssembly`
 
 Note that this is not intended to produce a production-ready output module
 but rather is intended purely as a temporary \"hack\" until it's standard in
@@ -38,6 +39,7 @@ struct Args {
     flag_output: Option<PathBuf>,
     flag_typescript: bool,
     flag_base64: bool,
+    flag_wasm2asm: bool,
     flag_fetch: Option<String>,
     arg_input: PathBuf,
 }
@@ -58,10 +60,6 @@ fn main() {
 }
 
 fn rmain(args: &Args) -> Result<(), Error> {
-    if !args.flag_base64 && !args.flag_fetch.is_some() {
-        bail!("unfortunately only works right now with base64 or fetch");
-    }
-
     let mut wasm = Vec::new();
     File::open(&args.arg_input)
         .and_then(|mut f| f.read_to_end(&mut wasm))
@@ -69,6 +67,7 @@ fn rmain(args: &Args) -> Result<(), Error> {
 
     let object = wasm_bindgen_cli_support::wasm2es6js::Config::new()
         .base64(args.flag_base64)
+        .wasm2asm(args.flag_wasm2asm)
         .fetch(args.flag_fetch.clone())
         .generate(&wasm)?;