Improve assets size

[lucleray]

It looks like :

Pages sizes after gzip:

┌ / (196 B)
├ /_app (11.5 kB)
├ /_error (4.44 kB)
├ /blog (196 B)
└ /blog/page (195 B)

(style inspired from now-cli : https://github.com/zeit/now-cli/blob/canary/src/util/output/builds.js)

I'll add dynamic chunks in a separate PR.

@timneutkens Do you want to keep _app and _error or filter them out ? I think it's a good idea to keep them, because _app can get pretty large and it would encourage code splitting in that case.

[timneutkens]

🏆 Looks perfect (taken from the test run in circle)