\usemodule[filter]
\defineexternalfilter[rest][filtercommand={pandoc -f rst -t context -o \externalfilteroutputfile\space \externalfilterinputfile}]
\starttext
There should be a table here.
\startrest
=========	=========
  test			table
=========	=========
α						b
c						d
========	========
\stoprest
\stoptext