Make $config:data-exclude extendable

[DrRataplan]

If you have static content, you want to exclude that collection. For example:

declare variable $config:data-exclude :=
    doc($config:data-root || "/taxonomy.xml")//tei:text,
    collection($config:register-root)//tei:text,
    collection($config:data-root || '/static')//tei:text

Move it to generated-config in base10, and read it from config?excluded-collections or something?