Use LoDash template with <</>> delimiters for sourceMapFilename.

The given filename string is processed as a [LoDash template](http://lodash.com/docs#template) with available objects:
* `cssFile`: Generated CSS full path.
* `cssName`: Generated CSS file name.
* `cssDir`: Generated CSS directory.
* `isLink`: True when used to link from generated CSS, false otherwise.
* `auto`: CSS file name when used to link, full path otherwise.
* `path`: Node.js [path module](http://nodejs.org/api/path.html).
* `grunt`: Grunt module.

To avoid processing by Grunt [config](http://gruntjs.com/api/grunt.config#grunt.config.init), the filename template uses `less` set of delimiters that defaults to `<<`/`>>`. The delimiters can be customized using Grunt template [addDelimiters method](http://gruntjs.com/api/grunt.template#grunt.template.adddelimiters): `grunt.template.addDelimiters('less', '{{', '}}');`.

Sample: `sourceMapFilename: '<<= auto >>.map'`

This is useful e.g. when generating [multiple CSS and source map files](http://gruntjs.com/configuring-tasks#files-object-format) in single task.