Automatically install plugins

I'm not sure if I want to do this yet, but maybe automatically npm install registered packages that haven't been installed. 

Another idea is to provide a way of specifying plugins on the tasks itself. Maybe something like:

``` javascript
guzzle.task('default').plugins('concat', 'uglify').src('src/*').concat().uglify.dest('./dist');
```

I'm a little less sure I want this though.
