Fluent API for Webpack

Webpack has a lot of features and can do a lot more than we usually use it for:

• Code Splitting

• Hashing

• CSS / SASS

Now, we don't need to use all of those, but some of them (like code splitting) are super useful.

Additionally, calling `mix.webpack()` more than once tends to create issues.

We can make a fluent webpack api to add simple tasks like Extract Text or File Loader or anything like that.

Currently, we could do this by including extensions. The `coldbox-elixir-vue-2` extension does just this. But this could be easier.

Additionally, we could help out multiple `mix.webpack()` calls by merging the calls into one before passing it on to webpack.

Finally, the docs for webpack need to be updated to show the current best practices for usage.