These changes allow empty public path in webpack.config.js

[stoccc]

This allows empty publicPath in webpack.config.js Provided a functional test. See https://github.com/symfony/webpack-encore/issues/26#issuecomment-362560685

I'm new to webpack/encore/nodejs/git, so I hope I didn't make any mistake :)

[stoccc]

@weaverryan I think I'll need some help with the functional test...

[stoccc]

It seems that we have some problems using javascript require.ensure: the file path is not translated to the client according to the information stored in manifest.json. Is there any config that we can edit so that the reference is generated properly?