Added argparse option --convert

This option provides mp3 conversion if mimetype of downloaded file is
not equal to "audio/mpeg".
Conversion is not processed here, there is a todo. This changes add
--convert option with argparse and create strack.process_tags(),
strack.convert() method with a bit of refactoring of strack.download() method.
Implemented mp3 conversion

Added pydub dependency for mp3 conversion.
Updated README
Output improvements