Copy all assets found in `sourceDir`

[chrispcampbell]

Currently we copy a hardcoded set of assets from the sourceDir during the build. For projects that have other assets, like custom JS or CSS files, we need to be able to copy and reference those files.

To solve this, instead of having hardcoded substitutions like ${BASE_CSS_PATH}, we will copy all non-HTML files found in the sourceDir, and then you can reference them like ${ASSET-base.css} in the HTML templates.