override default of Isabelle_Process, notably for PIDE export of "document.tex";

--- a/src/Pure/Thy/document_antiquotation.ML	Sat May 19 20:19:15 2018 +0200
+++ b/src/Pure/Thy/document_antiquotation.ML	Sat May 19 20:42:34 2018 +0200
@@ -159,7 +159,7 @@
 
 fun eval ctxt (opts, src) =
   let
-    val preview_ctxt = fold option opts ctxt;
+    val preview_ctxt = fold option opts (Config.put show_markup false ctxt);
     val _ = command src preview_ctxt;
 
     val print_ctxt = Context_Position.set_visible false preview_ctxt;