Added missing longhands

[honzatmn]

Added properties: background-repeat, background-position)

Puppeteer returns those when returning styles even when background-position-x/y is experimental...

[gilmoreorless]

As with #4, this repo relies on data found in published specifications. I did some digging around regarding the status of these properties and found the following:

• background-position as a shorthand is only defined in the CSS Backgrounds and Borders Module Level 4, which is at the Editor's Draft phase and contains a big warning that it's not ready for implementation. However, every browser already supports the shorthand, so this is one of those times when the specs are playing catch-up.
• background-repeat as a shorthand is not found in any specification. There is some speculation around making it a shorthand, but it's on hold indefinitely.

In light of that, I'm going to merge in a modified version of this PR which includes background-position (with a note about its status) but not background-repeat.

Thanks for the contribution, and for sparking this investigation. 😄