Add option to save the cover art fetched from the internet

[thaffenden]

As kunst is triggered on every song change, if you're listening to an album that you don't have the artwork saved for you end up fetching it online for each track.

I've added a new option to set when running kunst to save the downloaded cover.png file to the album directory meaning it will only be downloaded the first time, as the next tracks played from the album will be able to find the local file.

Example of it running fetching the cover art for the first track, but then using the existing cover art for the next tracks played from the same album:

[sdushantha]

Thank you for the PR! I currently do not have access to a Linux machine to test it out but once I do, I'll review this PR and hopefully get it merged :)