- Electron version: 1.3.6
- Operating system: Windows 7 x64
- Other: Microsoft Office 2013 (will make more sense in a bit)

`shell.openItem()` is a blocking call, it would be nice if it's async or has an async version to handle synchronous external application behavior.

Reproduce: 
Open a password-protected Microsoft Excel file, `shell.openItem()` will be blocking until the Password window is submitted or dismissed. Consequently Microsoft Word does not have this issue.

Related to #2761.

shell/common/api/atom_api_shell.cc
v8::Handle<v8::Value> val,
                     base::win::ShortcutOperation* out) {
    std::string operation;
    if (!ConvertFromV8(isolate, val, &operation))
      return false;
    if (operation.empty() || operation == "create")
      *out = base::win::SHORTCUT_CREATE_ALWAYS;
    else if (operation == "update")
      *out = base::win::SHORTCUT_UPDATE_EXISTING;
    else if (operation == "replace")
      *out = base::win::SHORTCUT_REPLACE_EXISTING;
    else
      return false;
    return true;
  }
};
}  
#endif
namespace {
void OnOpenExternalFinished(gin_helper::Promise<void> promise,
                            const std::string& error) {
  if (error.empty())
    promise.Resolve();
  else
    promise.RejectWithErrorMessage(error.c_str());
}
v8::Local<v8::Promise> OpenExternal(const GURL& url, gin::Arguments* args) {