Add configuration option to generate per-file header for aggregations

It would be nice to include an option for producing a filename header before each file in an aggregation.  Our project is using the <nocompress> option in our dev settings to aggregate but not compress/munge the files.  We hope to use the same property with another option that will aid in JS/CSS development by placing a filename header at the start of each aggregated file.

Example:
```xml
<aggregations>
    <aggregation>
        <includes>
            <include>**/A.css</include>
            <include>**/B.css</include>
            <include>**/C.css</include>
        </includes>
        <addFileHeaders>true</addFileHeaders>
        <output>${project.build.outputDirectory}/aggregated.css</output>
    </aggregation>
    <aggregation>
        <includes>
            <include>**/X.js</include>
            <include>**/Y.js</include>
            <include>**/Z.js</include>
        </includes>
        <addFileHeaders>true</addFileHeaders>
        <insertNewLine>true</insertNewLine>
        <output>${project.build.outputDirectory}/aggregated.js</output>
    </aggregation>
</aggregations>
```
Would produce:
```css
/* A.css */.a>.contents {}
/* B.css */.b>.contents {}
/* C.css */.c>.contents {}
```
```javascript
/** X.js */
var x = "contents";
/** Y.js */
var y = "contents";
/** Z.js */
var z = "contents";
```
And of course, the header could be created for minified files as well.