Coverage with nyc not mapped correctly

Related to #1, I can't seem to get the nyc coverage reports working correctly.

In this fork: https://github.com/mdvorscak/vue-test-utils-ava-example I added nyc, partially following https://github.com/avajs/ava/blob/master/docs/recipes/code-coverage.md .

If you checkout the fork and run `npm run unit` you'll see the branch coverage on List.Vue is not correct:

This appears to be related to the source map (considering there are no lines 18-20 in the source List.vue)

I've tried a whole bunch of combinations of setups from the ava code coverage recipe, and can't seem to get the coverage working with Vue. Any ideas on how to get the source maps working?