Added `options.commentMarker` for better compatibility with other plugins that use conflicting `build:<type>` comments.

The `build:<type>` comment marker used by this plugin is similar to (and conflicts with) comments used by other plugins (most notably [grunt-usemin](https://github.com/yeoman/grunt-usemin)). This PR removes this conflict by allowing you to use a word besides `build` in the opening/closing comments by setting the `commentMarker` option. The default value remains `build`, so this new option will not break any existing Gruntfiles.