The new `setAsDefaultProtocolClient`  and `removeAsDefaultProtocolClient` APIs are great, but without a way to query the current state, it's not possible to implement a UI element like this one. I think it'd be great to add `isDefaultProtocolClient`!

![image](https://cloud.githubusercontent.com/assets/1037212/14545184/fc1a6fac-026a-11e6-98d5-8bf6b2f9678e.png)

Also, I find the naming of these APIs pretty confusing. On Mac OS X and iOS `mailto` is referred to as a "URL Scheme", not a protocol (https://developer.apple.com/library/ios/featuredarticles/iPhoneURLScheme_Reference), and it seems like that might be the case on Windows too. (https://msdn.microsoft.com/en-us/library/aa767914(v=vs.85).aspx)

It'd be nice to clarify that this is registering the app system-wide, while the other `protocol` module is only for handling requests initiated from within the app. 

Maybe we could change these to `registerForURLScheme`, `isRegisteredForURLScheme`?

atom/browser/api/atom_api_app.cc
mate::Arguments* args) {
  base::string16 username, password;
  if (args->GetNext(&username) && args->GetNext(&password))
    login_handler->Login(username, password);
  else
    login_handler->CancelAuth();
}
#if defined(USE_NSS_CERTS)
int ImportIntoCertStore(
    CertificateManagerModel* model,
    const base::DictionaryValue& options) {
  std::string file_data, cert_path;
  base::string16 password;
  net::CertificateList imported_certs;
  int rv = -1;
  options.GetString("certificate", &cert_path);
  options.GetString("password", &password);
  if (!cert_path.empty()) {
    if (base::ReadFileToString(base::FilePath(cert_path), &file_data)) {
      auto module = model->cert_db()->GetPublicModule();
      rv = model->ImportFromPKCS12(module,
                                   file_data,
                                   password,
                                   true,
                                   &imported_certs);