Webpack config tweak: name output sourcemaps after the output file with ext.

[stevegeek]

The following is a proposed change that could make the default webpack config slightly more generic.

It is a small tweak to the installed config that changes the sourceMapFilename template to [file].map.

This makes things easier if the config is expanded to include css extraction say.

For example, if mini-css-extract-plugin is used with css-loader / sass-loader then the resulting source map is named correctly (application.css.map for example) without having to realise/remember the sourceMapFilename needs changing.

[rstacruz]

This is a great idea!