Introduce --link-module to ./configure

- Allows specifying a _third_party_main outside of the node repository
- Allows embedders to create custom builtin modules outside of node's
  repository

PR-URL: #2497
Reviewed-By: fishrock123 - Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: evanlucas - Evan Lucas <evanlucas@me.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>

configure

@@ -84,6 +84,13 @@ parser.add_option("--fully-static",
     help="Generate an executable without external dynamic libraries. This "
          "will not work on OSX when using default compilation environment")
 
+parser.add_option("--link-module",
+    action="append",
+    dest="linked_module",
+    help="Path to a JS file to be bundled in the binary as a builtin."
+         "This module will be referenced by basename without extension."
+         "Can be used multiple times")
+
 parser.add_option("--openssl-no-asm",
     action="store_true",
     dest="openssl_no_asm",
@@ -697,6 +704,9 @@ def configure_node(o):
   if options.enable_static:
     o['variables']['node_target_type'] = 'static_library'
 
+  if options.linked_module:
+    o['variables']['library_files'] = options.linked_module
+
 
 def configure_library(lib, output):
   shared_lib = 'shared_' + lib