added ability to use the entire grunt config in templates

I found that the only 2 properties that can be used are `file` and `meta`. No other grunt config variables are available.

My usecase is:
I use [grunt-prompt](https://github.com/dylang/grunt-prompt) to configure a setting file. User provides 3 values and grunt-prompt stores them in the grunt.config object.
I want to use them in grunt-generate then, but they're not processed.
Neither do values set in `grunt.config.init`

I took a look into the code and I found:
```javascript
    var data = {
      file : destination,
      meta : {
        className : _s.classify( destination.name ),
        type      : source.path.replace( '/', '.' ),
        package   : _s.camelize( destination.dir.replace( '/', '.', 'g' ) )
      }
    };
    data.meta.fqn = data.meta.package + '.' + data.meta.className;

    var processed = grunt.template.process( grunt.file.read( source.absolute ), {
      data : data
    } );
```

It does not look like any of the config variables are passed to the `grunt.template.process` explicitly.

After adding:
```
    var _ = require( 'underscore' );
    var processed = grunt.template.process( grunt.file.read( source.absolute ), {
      data : data : _.extend({}, grunt.config.data, data)
    } );
```
it works well.