Pass publicPath to Webpack dev server

[meghprkh]

This allows us to have the dev server running on for a subpath instead of the domain.

For example serving on localhost:8000/example instead of localhost:8000. This makes it run like it would run in the production environment.

[meghprkh]

Please put this PR on hold. Apparently for me webpack-dev-server is not working with historyApi Fallback when publicPath is given.

Ref https://github.com/webpack/webpack-dev-server/issues/216

[meghprkh]

Referencing https://github.com/webpack/webpack-dev-server/pull/518 .

Please wait till this gets merged

[olahol]

Nice work patching webpack-dev-server :+1:, will merge and update the dependencies then

[victorb]

@meghprkh this can also be set via setting the devServer key in webpack.config.js, since https://github.com/olahol/reactpack/pull/27 has been merged.