Allow for options to be passed into SVGO from Gruntfile

I know there is another pull request for this feature (just noticed it... sorry), but mine seems a lot simpler and works really well so far in my testing.   It allows for a Gruntfile such as this where `opts` are passed directly to the SVGO constructor:

```javascript
grunt.initConfig({
	svgo: {
		optimize: {
			files: 'svg/*.svg',
			opts: {
				config: './.svgo.yml',
				disable: 'cleanupNumericValues'
			}
		}
	}
}
```