Asset building

[groenroos]

To truly live up to the philosophy of zero config development, Sapling should probably also include automatic asset building for JS & CSS preprocessors out of the box.

There's no point is writing some brand new API for this, so we can likely get away with just creating a light wrapper around Laravel Mix. Potentially generate the mix file automatically (where the user does not provide their own) based on the contents of a magical assets/ folder?

[groenroos]

Moving this issue to the CLI, as this feels like something that probably shouldn't exist in the core by default, but is a good dependency to offer installation of in the CLI.