adding possibility to enter multiple "basePath" with Array.

This option allows you to reduce the search path in the archives. 

Directory structure: 
```
app
|-- brands
|   |-- myBrand
|   |   `-- index.dust
|   `-- otherBrand
|       |-- index.dust
|       `-- layouts
|           `-- base.dust
|-- index.dust
`-- layouts
    `-- base.dust
```

And Grunt config:
```
    dusthtml:{
      myBrand: {
        options: {
          basePath: ['app/brands/myBrand/', 'app/']
        },
        files: ['app/brands/myBrand/index.dust']
      },
      otherBrand: {
        options: {
          basePath: ['app/brands/otherBrand/', 'app/']
        },
        files: ['app/brands/otherBrand/index.dust']
      }
    }
```
Say ```app/brands/myBrand/index.dust``` and ```app/brands/otherBrand/index.dust``` contains:

```
{>"layouts/base"}
{<body}my body block{/body}
```

So ```app/brands/myBrand/index.dust``` read the file ```app/layouts/base.dust``` and ```app/brands/otherBrand/index.dust``` read ```app/brands/otherBrand/layouts/base.dust```