Provide an option to specify the filename for getBundleFileUrl

[bobmonsour]

The use case for this is when the bundle creation appears in a base layout and that layout it used for many of the site's templates. It would assist in avoiding recreating the bundle file or any specified post-bundle transform, such as minification. There are no doubt other ways to approach this, but I wanted to throw out this suggestion. The issue, however, that I imagine is that when bundling if one file or element of the bundle changes, that would require a new bundle output. One alternative to specifying the bundle filename might be for the bundle plugin itself to do a memoization of the result.