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
v8::Isolate* isolate,
    std::shared_ptr<content::ClientCertificateDelegate> delegate,
    mate::Arguments* args) {
  mate::Dictionary cert_data;
  if (!args->GetNext(&cert_data)) {
    args->ThrowError();
    return;
  }
  v8::Local<v8::Object> data;
  if (!cert_data.Get("data", &data))
    return;
  auto certs = net::X509Certificate::CreateCertificateListFromBytes(
      node::Buffer::Data(data), node::Buffer::Length(data),
      net::X509Certificate::FORMAT_AUTO);
  if (certs.size() > 0)
    delegate->ContinueWithCertificate(certs[0].get());
}
void PassLoginInformation(scoped_refptr<LoginHandler> login_handler,