Rewrite vue-loader configs

[LinusBorg]

The current config is pretty complicated, if not downright cryptic, for new users.

It is that way because we wanted needed to incorporate quite a few options to make all cases (css sourcemaps: yes/no - extract-css: yes/no) work together, without too much code duplication.

But most of those things are now options that are avaiable in vue-loader itself, so the configuration could be much, much simpler.

This still leaves the configuration for the normal preprocessor loaders (e.g.for actual .scss or .less files. I intend to redo those too, and make them more explicit and easy to follow, even if that means to duplciate a few pieces of code. but maybe there's not too much to do after all, we'll see.

I will work on this after #975 has been is published.