add highlight modification

[emmamorlock]

I needed a highlight with stripes (to highlight witnesses in a webapp with a critical apparatus), similar to the one which is used with the annotation interface. In my use case, a witness is associated with 2 categories, represented by 2 different colors, hence the need for stripes combining the same 2 colors.

Gradients css can produce that easily, but they need a "background-image" property.

I just added a property to the component --pb-highlight-background-image to be exposed in the api. I added a demo which shows a highlighight with stripes.

PS. I also modified the @cssprop for the initial --pb-highlight-color to indicate the color code of the default color (#F9E976).