Support new svg format of simple-icons

[yizeng]

https://simpleicons.org/

[pkeir]

I created a bash script to download and scale svg icons from simple-icons.org, before converting them to png, and outputing a SassScript (CSS) fragment with background colour information; suitable for insertion into your _social-icons.scss file. I'm working under Ubuntu. 18.10. Here's an example invocation:

bash simple-icons-png.sh strava bitbucket "Adobe Acrobat Reader"

I've put the script in a gist here.