Fix for using sources with a map.

sources.map() was giving map undefined when using this style:
'target2.html': {
          file: 'source.hbs.html',
          context: {
            key: 'value'
          }
        }

Fix is to first see if the source has a file attribute and use that.
This has the upside of allowing an array of sources even when using
the above notation.