Update default extensions to match node

Extension list sourced from https://nodejs.org/api/modules.html#modules_file_modules

Fixes #137

Author: TehShrike

Diff for: lib/async.js

@@ -29,7 +29,7 @@ module.exports = function resolve(x, options, callback) {
     };
     var readFile = opts.readFile || fs.readFile;
 
-    var extensions = opts.extensions || ['.js'];
+    var extensions = opts.extensions || ['.js', '.json', '.node'];
     var y = opts.basedir || path.dirname(caller());
 
     opts.paths = opts.paths || [];

Diff for: lib/sync.js

@@ -20,7 +20,7 @@ module.exports = function (x, options) {
     };
     var readFileSync = opts.readFileSync || fs.readFileSync;
 
-    var extensions = opts.extensions || ['.js'];
+    var extensions = opts.extensions || ['.js', '.json', '.node'];
     var y = opts.basedir || path.dirname(caller());
 
     opts.paths = opts.paths || [];

Diff for: readme.markdown

@@ -84,7 +84,7 @@ default `opts` values:
 {
     paths: [],
     basedir: __dirname,
-    extensions: [ '.js' ],
+    extensions: [ '.js', '.json', '.node' ],
     readFile: fs.readFile,
     isFile: function (file, cb) {
         fs.stat(file, function (err, stat) {
@@ -132,7 +132,7 @@ default `opts` values:
 {
     paths: [],
     basedir: __dirname,
-    extensions: [ '.js' ],
+    extensions: [ '.js', '.json', '.node' ],
     readFileSync: fs.readFileSync,
     isFile: function (file) {
         try { return fs.statSync(file).isFile() }