Free DeepSpeech resources

[rtoepfer]

I didn't compile and test this but plan to in the future if I have the time.

See https://github.com/mozilla/DeepSpeech/blob/master/native_client/deepspeech.h

   * @param[out] aMFCC An array containing features, of shape
   *                   (@p aNFrames, ncep * ncontext). The user is
   *                   responsible for freeing the array.

   * @return The resulting string after running inference. The user is
   *         responsible for freeing this string.