Add a way to get default settings

- Implement a means of obtaining default settings for an
   input/output/encoder.  obs_source_defaults for example will return
   the default settings for a particular source type.

 - Because C++ doesn't have designated initializers, use functions in
   the WASAPI plugin to register the sources instead.

libobs/obs-encoder.h

@@ -134,6 +134,13 @@ struct obs_encoder_info {
 	/* ----------------------------------------------------------------- */
 	/* Optional implementation */
 
+	/**
+	 * Gets the default settings for this encoder
+	 *
+	 * @param[out]  settings  Data to assign default settings to
+	 */
+	void (*defaults)(obs_data_t settings);
+
 	/** 
 	 * Gets the property information of this encoder
 	 *