Add feature to extract cover image

[qubbit]

ref: qubbit@2ccd638

[jkongie]

Hi! I'll look into this.

Cheers

[qubbit]

I am sorry that my code looks like it was written in a hurry, and it was! I was making a proof-of-concept application in rails. But I think this is a good feature to add. The code I added is based on this answer from SO.

[tomash]

This is pretty cool but I'd remove the adding of pry gem.

(yes, I just stumbled upon this gem, hence thread necromancy)