Transfer retina logic into `spritesmith`

[twolfson]

This is from a few months old note:

We should take the retina support from grunt-spritesmith and gulp.spritesmith. However, the filtering that they do should be left to them. We should make this a simpler API which allows for src and retinaSrc to be passed in pre-filtered.

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We might want to make this a separate method from spritesmith's core to allow the library to stay focused on one thing well.