Add artwork for AAC (M4A) audio

A long-standing limitation with Pinecast has been the inability to add custom artwork for M4A audio files. We’ll now prompt you to add artwork for these files.

As a note, our new M4A support doesn’t yet detect embedded images, and we aren’t about to embed the chosen image in the M4A metadata. This change affects the image tags in feeds and artwork on podcast websites, players, and short links.