Add config support to export project with custom path prefix

[moarwick]

Currently, next export generates files assuming absolute paths. This is problematic when the static site is pushed to a sub-folder, e.g. Github Pages.

For example, if I push my my-project/out folder to a gh-pages branch, then access it at https://user.github.io/my-project/, the page is looking for assets at /_next/..., for example href="/_next/a3c4359f-2b55-4680-b981-dfddcf84cea4/page/index.js"

To solve the problem, I think we want to be able to specify a custom path e.g. /my-project, so then it would look for assets in /my-project/_next/...

[sergiodxa]

You already can do that using assetPrefix configuration. Just add that key to your next.config.js and set /my-project if you are running on production, that way when running next build you will have /my-project/_next/... and in development you will use /_next/....

[arunoda]

@moarwick deploying any Next.js app into a sub directory is kind a tricky. We'll address this after 3.0