support custom coloring on amp-social-share SVG icon fills

According to https://amp.dev/documentation/examples/components/amp-social-share/?format=websites, icons colors can be styled using a css color attribute, same as the background-color attribute, however on the invoke of registerElement in amp-social-share-0.1.js the svg fill attribute of the default icons is fixed to '#fff' making the color attribute useless.

@paul-matthews

Start in amp-social-share-0.1.js at registerElement and inspect how the default SVG icons’ fill is set. Compare this with the amp-social-share documentation example’s color and background-color styling. Done means the color attribute can control the default icon fill instead of it remaining fixed at #fff.

Written by the indexing model from the issue text.