Concatenated assets

[jace]

Assets can be specified using assets_js and assets_css in options. These are currently a list of string paths, and the assets are minified but not concatenated, for usecases where an asset specifies a path relative to itself and cannot be moved or rewritten safely (Leaflet.js does this).

It should be possible to ask for concatenated assets by using a sub-list, like so:

{
assets_css: [
  "asset1",
  ["asset2a", "asset2b", "asset2c"],
  "asset3"
  ]
}

In this case, assets 2a, 2b and 2c should be concatenated into a single asset2.