List support for doc_root and source_ext options.

doc_root and source_ext can now receive lists as parameters. This is
particularly useful when using ErlyDTL if you want to have multiple
extensions (.html, .xml, ...) in many locations. Example :

    {erlydtl opts, [
        {doc_root, ["xml", "html"]},
        {source_ext, [".xml", ".html"]},
    ]}.

Note that the current format (with just a string) is still supported.