Add twig icon macro/helper to Craft setup

[greypants]

Lets provide a sprite helper for use with the svgSprite task for Craft, like we do for Rails and the default Nunjucks init tasks.

Should be pretty similar to https://github.com/vigetlabs/blendid/blob/master/src/html/macros/helpers.html#L1-L6 and https://github.com/vigetlabs/blendid/blob/master/extras/rails/app/helpers/blendid_asset_helper.rb#L24-L31

I don't think we need the wrapper span anymore... I believe that was to work around a cross-browser issue with IE 9 or something.