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
auto command_line = base::CommandLine::ForCurrentProcess();
  if (switch_string == atom::switches::kPpapiFlashPath ||
      switch_string == switches::kLogNetLog) {
    base::FilePath path;
    args->GetNext(&path);
    command_line->AppendSwitchPath(switch_string, path);
    return;
  }
  std::string value;
  if (args->GetNext(&value))
    command_line->AppendSwitchASCII(switch_string, value);
  else
    command_line->AppendSwitch(switch_string);
}
#if defined(OS_MACOSX)
int DockBounce(const std::string& type) {
  int request_id = -1;
  if (type == "critical")
    request_id = Browser::Get()->DockBounce(Browser::BOUNCE_CRITICAL);
  else if (type == "informational")
    request_id = Browser::Get()->DockBounce(Browser::BOUNCE_INFORMATIONAL);
  return request_id;
}
void DockSetMenu(atom::api::Menu* menu) {
  Browser::Get()->DockSetMenu(menu->model());
}
#endif
void Initialize(v8::Local<v8::Object> exports, v8::Local<v8::Value> unused,