added support for custom models

[ASoggyToaster]

Added support for custom models - they require a customization_id value to be added to the connection url.

A sample of how they were used in my code is shown below.

	file := strings.ToLower(e)
	encoding := ""
	model := ""
	customization_id := ""

	if strings.Contains(file, "mulaw") {
		encoding = "audio/mulaw;rate=8000"
		model = "en-US_NarrowbandModel"
		customization_id = "802d10d0-80c1-11e7-83e1-49cd271ae6a2"
	} else if strings.Contains(file, "linear16") {
		encoding = "audio/wav"
		model = "en-US_BroadbandModel"
		customization_id = "42f6e650-80c1-11e7-83e1-49cd271ae6a2"
	} else if strings.Contains(file, "flac") {
		encoding = "audio/flac"
		model = "en-US_BroadbandModel"
		customization_id = "42f6e650-80c1-11e7-83e1-49cd271ae6a2"
	} else {
		continue
	}

	evt, writer, err := client.NewStream(model, customization_id, encoding, make(map[string]interface{}))

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