Would like the ability to disable use of iSpeech SDK for iOS 9 and older

[wifisher]

Starting with iOS 10, this plugin uses the native SFSpeechRecognizer for speech recognition. The iSpeech SDK is used for iOS 9 and older.

Since most iOS devices in use are now iOS 10 or newer, it would be nice to be able to disable the use of the iSpeech SDK and not have to set up an account with them, etc.

I propose renaming the existing config.xml preference from apiKey to speechRecognitionApiKey while keeping support for apiKey for backwards compatibility. I also propose that setting speechRecognitionApiKey to disable would prevent the plugin from using the iSpeech SDK.

I will include these changes in a pull request.