Cater for asset types besides code/style

[jpap]

I'm a big fan of the brunch.io application assembler, where you can specify an assets folder tree, where everything inside gets copied to the public folder.

It'd be great if we can configure an input folder name for such a copy.

This means that we can:

1. wipe out the public folder to "clean" the build at any time without having to worry about deleting "real" assets.
2. simply add the public folder to our .gitignore file.