Add support to platform_img() for additional platforms beyong the default 'mac', 'linux', and windows (i.e. blank). This allows adding more specific platform images when we have them available, such as the screenshots on the firefox/new page and is similar to the functionality from the old buildPlatformImage() function $platforms array. Here we add additional platforms via the <img data-additional-platforms="fxos ios"> attribute.
For now, the iOS image is the same as the android image. See https://bugzilla.mozilla.org/show_bug.cgi?id=883808#c3
Merge pull request #3 from nburka/bug-868199-unified-download

platform_img() now supports optional additional platforms - and add an iOS image for the /new page