Add ability to add arbitrary config options at site level

[paltman]

This should allow passing in configuration values that can be available to widgets that vary from site to site.

Usage

const config = {
  mapAccessToken: 'XXXXXXXXX',
};
Vue.use(SkeletonPlugin, { iconMap, config });

Then in the widget:

  computed: {
      accessToken() {
        return this.$scaife.config.mapAccessToken;
      },
  }