VIM: Support attachfile

[adityam]

It may be useful to support attach=yes for code snippets

\setupinteraction [state=start]
\starttext
  \attachment[file=attachfile.tex, title=Some file, author=Me]
\stoptext

[adityam]

\startsetups attachfile
  \expanded{\useattachment[\externalfilterparameter\c!reference]
                [\externalfilterparameter\c!title]
                [\externalfilterparameter\c!reference]
                [\externalfilterinputfile]}
\stopsetups

and then add

\defineexternalfilter
     [...]
     [...
        filtersetups={attachfile},
     ...]