Define an interceptor that is also an asset pipeline Consumer. Serves assets from the
associated pipeline as HTTP resources.

Arguments are:

- arachne id (optional) - the Arachne ID of the interceptor component
- options (optional) - A map (or kwargs) of additional options.

Currently supported options are:

- :priority - the priority relative to other interceptors defined at the same path. If omitted,
defaults to the lexical order of the config script
- :index? - Sets to true if requests to a directory name should attempt to resolve an
index.html file within the directory.

Note that the asset interceptor should always be installed to the server root; attachment to
other points in the routing table is (currently) not supported. This function should not be
called inside a `http/context`.