Twenty Sixteen: Add gradient background options that fit the theme color scheme

[ianbelanger]

Now that we have the option for certain blocks to have gradient backgrounds, we need to add gradient background options to the theme that fit the themes color scheme.

[tomjdevisser]

Added theme support and four gradients

[tomjdevisser]

For this one there's also mostly black/white, so I went with the WordPress colors that would match anything existing