Svg fixes

@drsnyder 
https://wikia-inc.atlassian.net/browse/PLATFORM-563

Add original file extension as a hint to `identify`. This should help with SVGs that are missing the xml header, and does not interfere with actual detection based on identification bytes (so `identify image.png.svg` properly identifies as a png file).

Also adding `THUMB_PRE_OPTIONS` to `bin/thumbnail`. This is to maintain image transparency for svg images. The location of `-background none` seems to matter, so we can't just prepend it to `THUMB_PARAMS` :(