Fix options loading and turn into MultiTask

Options loading didn't seem to work since `grunt.config('behat'` doesn't return anything.
This switches to using `this.options()` within the task. It also users `this.filesSrc` since grunt handles the glob automatically. This changes the config format to something more like this:

```
		behat:
		{
			core :
			{
				files : { src: 'application/tests/features/**/*.feature' },
				options :
				{
					config: 'application/tests/behat.yml',
					bin: './bin/behat',
					maxProcesses: 1,
				}
			}
		},
```