Process functional single file components

[agualis]

Vue supports Pure Functional templates from version 2.5:
https://vue-loader.vuejs.org/en/features/functional.html

With this pull request jest-vue would allow them to be tested.

• Note that we need to nest props attribute in propsData like this:

const wrapper = shallow(FunctionalSFC, {
    propsData: { props: { msg: 'Hello' }}
 })