Implement a js_name customization

This'll allow binding multiple signatures of a JS function as well as otherwise
changing the name of the JS function you're calling from the Rust function that
you're defining.

Closes #72

examples/console_log/src/lib.rs

@@ -8,9 +8,15 @@ use wasm_bindgen::prelude::*;
 extern {
     #[wasm_bindgen(js_namespace = console)]
     fn log(s: &str);
+    #[wasm_bindgen(js_namespace = console, js_name = log)]
+    fn log_u32(a: u32);
+    #[wasm_bindgen(js_namespace = console, js_name = log)]
+    fn log_many(a: &str, b: &str);
 }
 
 #[wasm_bindgen]
 pub fn run() {
     log("Hello from Rust!");
+    log_u32(42);
+    log_many("Logging", "many values!");
 }