Expose the AST node visitor builder via PEG.compiler.visitor

This is useful mainly for plugins which manipulate the AST.

lib/compiler.js

@@ -4,6 +4,12 @@ var arrays  = require("./utils/arrays"),
     objects = require("./utils/objects");
 
 var compiler = {
+  /*
+   * AST node visitor builder. Useful mainly for plugins which manipulate the
+   * AST.
+   */
+  visitor: require("./compiler/visitor"),
+
   /*
    * Compiler passes.
    *