Update karma.config.js to work with CSS preprocessors
Update karma.config.js to work with CSS preprocessors

This is a fix to a problem that I ran into when I tried to run unit tests with `<style lang='scss'>` in the `Hello.vue` component.
jmv-team/jmv-cli,neonblack-at/webpack,bbrietzke/webpack,neonblack-at/webpack,Brandon-Stoyles/webpack,twcapps/vue-typescript-boilerplate,Toilal/vue-webpack-template,jveljan/webpack,jmv-team/jmv-cli,twcapps/vue-typescript-boilerplate,Toilal/vue-webpack-template,twcapps/vue-typescript-boilerplate,Toilal/vue-webpack-template,twcapps/vue-typescript-boilerplate,bbrietzke/webpack,vuejs-templates/webpack,pronebel/webpack,jveljan/webpack,Brandon-Stoyles/webpack,vuejs-templates/webpack,neonblack-at/webpack,bbrietzke/webpack,jmv-team/jmv-cli,Toilal/vue-webpack-template,pronebel/webpack,vuejs-templates/webpack,pronebel/webpack,Brandon-Stoyles/webpack,jveljan/webpack