Add support for Retina (@2x) comics

[thebluepotato]

Hi!

I reviewed the latest version of the app (2.5) asking to add support for @2x webcomics. Indeed, Randall has always provided an @2x version of the comic beginning with 1084 (see here). They're just the standard img URL with _2x appended before the extension. One could easily check that the comic is 1084 or later and also check the PNG actually exists (in case it's missing or it's a special comic).

Cheers!