Next.js: `next build` should have a progress indicator similar to `next export`

[Timer]

Feature request

At a minimum, running next build should give the user some sort of feedback on progress being made similar to next export.

Right now, the user is in the dark while we're running getStaticPaths and then getStaticProps for every returned path.

It'd be really cool to show this Jest test runner style, then automatically float long-running pages to the top (to identify bottle necks).

However, we could start with a basic version that just listed % complete and number remaining, plus warnings for any individual paths that are taking more than 10 seconds to execute (to identify hangs).

[gelbartj]

I've submitted a PR for this, with a basic # complete vs number remaining, using the same progress bar as next export.