New `watch` option pointing directly at Eleventy output folder will trigger two reload events

[zachleat]

One from Eleventy, one from the dev server.

e.g. watch: ["_site"]

Follow up to #19.

https://github.com/11ty/eleventy-dev-server/blob/ca8c8e9a8664f60551816ef86ca2f69d73cc8005/server.js#L82