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
base::Bind(&Browser::ClearRecentDocuments, browser))
      .SetMethod("setAppUserModelId",
                 base::Bind(&Browser::SetAppUserModelID, browser))
      .SetMethod("setAsDefaultProtocolClient",
                 base::Bind(&Browser::SetAsDefaultProtocolClient, browser))
      .SetMethod("removeAsDefaultProtocolClient",
                 base::Bind(&Browser::RemoveAsDefaultProtocolClient, browser))
#if defined(OS_MACOSX)
      .SetMethod("hide", base::Bind(&Browser::Hide, browser))
      .SetMethod("show", base::Bind(&Browser::Show, browser))
#endif
#if defined(OS_WIN)
      .SetMethod("setUserTasks",
                 base::Bind(&Browser::SetUserTasks, browser))
#endif
      .SetMethod("setPath", &App::SetPath)
      .SetMethod("getPath", &App::GetPath)
      .SetMethod("setDesktopName", &App::SetDesktopName)
      .SetMethod("allowNTLMCredentialsForAllDomains",
                 &App::AllowNTLMCredentialsForAllDomains)
      .SetMethod("getLocale", &App::GetLocale)
#if defined(USE_NSS_CERTS)
      .SetMethod("importCertificate", &App::ImportCertificate)
#endif
      .SetMethod("makeSingleInstance", &App::MakeSingleInstance);
}
}  
}  
namespace {
void AppendSwitch(const std::string& switch_string, mate::Arguments* args) {