Issue title: Can it be saved into MP3 or opus 
 Issue body: Can the output of tts be saved into MP3 or opus format?
 Comments: 
 Comment 0: The output format is dependent on the speech engine used. For example, the Windows output format is WAV. Conversion would be necessary to get an MP3 file.
 Comment 1: see #76 for more info  
{'source': 'GithubIssues', 'repo_name_with_owner': 'nateshmbhat/pyttsx3', 'issue_id': 545263581, 'repo_stars': 1220.0}