Add Source Sans Pro variable font

[elliottslaughter]

Source Sans Pro has had a variable font for at least a couple of years, but it is not available on Google Fonts. You can see the variable font right on the Github page:

https://github.com/adobe-fonts/source-sans-pro

Would it be possible to add? The fixed weight versions of the font are oddly inconsistent, which would make the variable font particularly nice.

Thanks!

[thlinard]

Duplicate of #1744.

[elliottslaughter]

Huh, I guess my search skill fail me. Thanks!