Added ability to register 6to5 with a blacklist

Currently, to achieve this, I have to use this workaround:

```js
var to5 = require('6to5')
delete to5.transform.transformers.generators
require('6to5/register')
```

After this simple change, I can make it much nicer:

```js
require('6to5/register')({
    blacklist: ['generators']
})
```

Cheers!

lib/6to5/register.js

@@ -21,6 +21,7 @@ sourceMapSupport.install({
 //
 
 var ignoreRegex = /node_modules/;
+var blacklist   = [];
 var exts        = {};
 var maps        = {};
 var old         = require.extensions[".js"];
@@ -31,7 +32,8 @@ var loader = function (m, filename) {
   }
 
   var result = to5.transformFileSync(filename, {
-    sourceMap: true
+    sourceMap: true,
+    blacklist: blacklist
   });
 
   maps[filename] = result.map;
@@ -39,6 +41,10 @@ var loader = function (m, filename) {
   m._compile(result.code, filename);
 };
 
+var addToBlacklist = function (array) {
+  blacklist = _.union(blacklist, array)
+}
+
 var hookExtensions = function (_exts) {
   _.each(exts, function (old, ext) {
     require.extensions[ext] = old;
@@ -63,4 +69,6 @@ module.exports = function (opts) {
   }
 
   if (opts.extensions) hookExtensions(opts.extensions);
+
+  if (opts.blacklist) addToBlacklist(opts.blacklist);
 };