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!