MPIDE-27: add flag to return original AST

Allows the user to specify whether the parse result should include the
original AST in addition to the normal model. Defaults to false.

src/lib/parser/modellang.pegjs

@@ -3,7 +3,8 @@
     var opts = arguments[2] || options,
         fileName = opts.fileName || "NOFILENAME",
         locations = opts.locations !== undefined ? opts.locations : true,
-        comments = opts.comments !== undefined ? opts.comments : false;
+        comments = opts.comments !== undefined ? opts.comments : false,
+        showAst = opts.showAst !== undefined ? opts.showAst : false;
 
     var err = function err(msg, suffix, opts) {
         opts = opts || {};
@@ -39,6 +40,8 @@
             init: [],
         };
 
+        if (showAst) model.ast = statements;
+
         // accumulate varying statements into a flatter structure
         for (var s in statements) {
             var statement = statements[s];