Added bitrate and extra parameters options to AudioSegment.export()

Optional bitrate passed to ffmpeg as -b. AudioSegment.export() can now also pass an arbitrary extra list of export parameters to ffmpeg using the 'parameters' option, in same format as used within the method. No validation of these parameters takes place.
New export ffmpeg options documented.
Merge pull request #13 from ptpittman/master

Additional options for mmpeg in AudioSegment.export()