add extra options to just return samples array without saving file

[casiodk]

This PR gives you the option to

1. not draw and save the png
2. just have the samples array returned, so i can use it for drawing a html5 canvas waveform

[casiodk]

@benalavi

[casiodk]

Hey @benalavi What do you think of this option. I need the samples array in order to create a html canvas waveform :)