Fix for using sources with a map.

[jeremyk]

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.

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