Allow strings to be concatenated along with files

I needed to be able to concatenate (dynamic) strings as well as files. Opening a pull request in case it's helpful for anyone.

### Example usage:

**Gruntfile.js**

    concat: {
      example: {
        files: {
          'my.txt': [
            'here is a string',
            'middle.txt',
            'here is another string'
          ],
        }
      }
    }

**middle.txt**

    this is the middle text file

**my.txt**

    here is a string
    this is the middle text file
    here is another string