Plugin API: Implement the --plugin option

Implements part of GH-106.

README.md

@@ -77,6 +77,8 @@ You can tweak the generated parser with several options:
     time in pathological cases but making the parser slower
   * `--allowed-start-rules` — comma-separated list of rules the parser will be
     allowed to start parsing from (default: the first rule in the grammar)
+  * `--plugin` — makes PEG.js use a specified plugin (can be specified multiple
+    times)
 
 ### JavaScript API
 

bin/pegjs

@@ -2,6 +2,7 @@
 
 var util = require("util");
 var fs   = require("fs");
+var path = require("path");
 var PEG  = require("../lib/peg");
 
 /* Helpers */
@@ -31,6 +32,8 @@ function printHelp() {
   util.puts("                                     grammar)");
   util.puts("  -o, --optimize <goal>              select optimization for speed or size (default:");
   util.puts("                                     speed)");
+  util.puts("      --plugin <plugin>              use a specified plugin (can be specified");
+  util.puts("                                     multiple times)");
   util.puts("  -v, --version                      print version information and exit");
   util.puts("  -h, --help                         print help and exit");
 }
@@ -75,7 +78,8 @@ var exportVar = "module.exports";
 var options = {
   cache:    false,
   output:   "source",
-  optimize: "speed"
+  optimize: "speed",
+  plugins:  []
 };
 
 while (args.length > 0 && isOption(args[0])) {
@@ -115,6 +119,22 @@ while (args.length > 0 && isOption(args[0])) {
       options.optimize = args[0];
       break;
 
+    case "--plugin":
+      nextArg();
+      if (args.length === 0) {
+        abort("Missing parameter of the --plugin option.");
+      }
+      var id = /^(\.\/|\.\.\/)/.test(args[0]) ? path.resolve(args[0]) : args[0];
+      try {
+        var module = require(id);
+      } catch (e) {
+        if (e.code !== "MODULE_NOT_FOUND") { throw e; }
+
+        abort("Can't load module \"" + id + "\".");
+      }
+      options.plugins.push(module);
+      break;
+
     case "-v":
     case "--version":
       printVersion();