Extend contentBase to support array with multiple sources.

[BerndWessels]

Hello

This is a small but important PR that will allow us to have multiple sources as content base.

It opens a raft of development build features and solves masses of messy workarounds to achieve the same thing.

One typical example on how it would be used in a webpack.config is:

var WHITELABEL = 'visa';
config.devServer = {
  contentBase: ['./public/common', './public/' + WHITELABEL]
}

In this example we are now able to separate common and whitelabel specific assets, externals and sources cleanly in different folders - rather than having copies of all common files in each whitelabel folder.

[sokra]

Thanks