Allow passing of template folder path as a string argument

Currently, it has to be passed as an array to work:
```js
nunjucks: {
  options: {
     paths: ['templates'],
     data: grunt.file.readJSON('results.json')
   }, 
   ...
}
```
 We can specify paths as an array or a string, so I think it would be better to check like this:

```
if (options.path) {
  nunjucks.configure(options.path);
}
```

What do you think?