Implement a version string

Add a `--version` and `-V` to the command to print out the version

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

@@ -2,6 +2,7 @@ extern crate wasm_bindgen_cli_support;
 #[macro_use]
 extern crate serde_derive;
 extern crate docopt;
+extern crate wasm_bindgen_shared;
 
 use std::path::PathBuf;
 
@@ -14,6 +15,7 @@ Generating JS bindings for a wasm file
 Usage:
     wasm-bindgen [options] <input>
     wasm-bindgen -h | --help
+    wasm-bindgen -V | --version
 
 Options:
     -h --help               Show this screen.
@@ -21,6 +23,7 @@ Options:
     --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
 ";
 
 #[derive(Debug, Deserialize)]
@@ -29,7 +32,8 @@ struct Args {
     flag_typescript: bool,
     flag_out_dir: Option<PathBuf>,
     flag_debug: bool,
-    arg_input: PathBuf,
+    flag_version: bool,
+    arg_input: Option<PathBuf>,
 }
 
 fn main() {
@@ -37,8 +41,18 @@ fn main() {
         .and_then(|d| d.deserialize())
         .unwrap_or_else(|e| e.exit());
 
+    if args.flag_version {
+        println!("wasm-bindgen {}", wasm_bindgen_shared::version());
+        return
+    }
+
+    let input = match args.arg_input {
+        Some(s) => s,
+        None => panic!("input file expected"),
+    };
+
     let mut b = Bindgen::new();
-    b.input_path(&args.arg_input)
+    b.input_path(&input)
      .nodejs(args.flag_nodejs)
      .debug(args.flag_debug)
      .typescript(args.flag_typescript);