Add args option to gitcommit task

Add the ability to append additional arguments to commit commands.

Example usage:

```js
grunt.initConfig({
    gitcommit: {
        task: {
            options: {
                message: 'Testing',
                args: ['--no-verify', '--no-status']
            },
            files: {
                src: ['test.txt']
            }
        }
    },
});
```

Result:

```
git commit -m 'testing' --no-verify --no-status
```