Optionally allow to explicitly specify Cover Image format

[hXtreme]

On going through some of the downloaded files, I found out that some files contain cover image in jpg while others in png fomat. This may be due to how the files are on SoundCloud, haven't investigated.

So in the wake of problems referred in various other issues, #184, #183 we can add an optional parameter to tell how we want the cover image to be stored.

As a starting point please refer, from 183