Add wrapper option.

This option allows for the concatenated files to be wrapped, like in grunt-wrap. For example:

```js
// file_a
return 'A';
```
```js
// file_b
return 'B';
```
```js
// excerpt Gruntfile.js
concat: {
  build: {
    src: ['file_a.js', 'file_b.js'],
    dest: 'concat.js'
    options: {
      wrapper: ['module.exports = function() { \n', '\n};']
     }
  }
}
```
Result:
```js
module.exports = function() {
// file_a
return 'A';
};
module.exports = function() {
// file_b
return 'B';
};
```