Custom indentation can be specified in the Gruntfile

This patch adds the option to specify the indentation in your Gruntfile as per issue #3. Currently four spaces are used.

The indent value is optional and if unspecified the default of four space will be applied.

Indentation can be specified in your Gruntfile like so.

```javascript
bower: {
  rjsConfig: 'scripts/config.js',
  // Use 2 spaces for indentation (default is 4)
  indent: '  '
}
```

I've also added the option to the readme file.