Fields for background size and attachment

[herbdool]

A couple fields to set more background css parameters would be useful. I've already customized my local copy to do so.

• Background size: "contain" or "cover". I like to set it to cover in some cases so that it works better on mobile if there is lots of text.
• Background attachment: allow for "fixed" or default (scroll). I found the parallax effect doesn't work how I expected, whereas "fixed" will keep the background fixed to the viewport while content scrolls up.