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
content::WebContents* web_contents,
    int cert_error,
    const net::SSLInfo& ssl_info,
    const GURL& request_url,
    content::ResourceType resource_type,
    bool overridable,
    bool strict_enforcement,
    bool expired_previous_decision,
    const base::Callback<void(bool)>& callback,
    content::CertificateRequestResultType* request) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit("certificate-error",
                              WebContents::CreateFrom(isolate(), web_contents),
                              request_url,
                              net::ErrorToString(cert_error),
                              ssl_info.cert,
                              callback);
  
  if (!prevent_default)
    *request = content::CERTIFICATE_REQUEST_RESULT_TYPE_DENY;
}
void App::SelectClientCertificate(