Hi,

I like to achieve something that looks like this:
```
\definedelimitedtext[extract][blockquote]
\setupdelimitedtext
  [extract]
  [leftmargin=1.5pc,
   style={\italic},
   before={\setupindenting[next]},
   after={\blank[1ex] \hrule \blank[1ex] \startalignment[flushright] \tfx\italic{René Descartes} \stopalignment}]

\starttext
\startextract
    Cogito ergo sum.
\stopextract
\stoptext
```

So it should setup a quotation and mentions the author.
However, here the author is hardcoded within the blockquote. I would like it to use like this:
```
\definedelimitedtext[extract][blockquote]
\setupdelimitedtext
  [extract]
  [leftmargin=1.5pc,
   style={\italic},
   before={\setupindenting[next]},
   after={\blank[1ex] \hrule \blank[1ex] \startalignment[flushright] \tfx\italic{\getcustomvariable{author}} \stopalignment}]

\starttext
\startextract[author=René Descartes]
    Cogito ergo sum.
\stopextract
\stoptext
```

Is there an easy way to achieve that?
I tried with \structureuservariable (like possible in \startchapter) but it does not work.

Best,
Gerion