feat(widget): add `css_class` option to customize widget style
feat(widget): add `css_class` option to customize widget style

Allows user to customize the style of a particular widget using the CSS
plugin feature. For example, the background of a widget could be changed
to an image or gradient.

Close #322

gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic