flag to return only file index

[felixbur]

I would assume that very often people simply use this package to extract acoustic features per file as opposed to low level features.
Problem is that the process_files() method returns a multiindex with start and end which doesn't really makes sense if it' for the whole file anyway.
I know I can drop the superflous levels, but I'd appreciate a flag like 'multiindex==False' or so.