Adds the option to specify files that need a third pass of UseRev

[MiguelMadero]

Sort-of FIXES https://github.com/rickharrison/broccoli-asset-rev/issues/29 (or more like works around it).

For cases where you know which files have references to other files that are in the replaceExtensions (by default JS, HTML, CSS) we can explicitly add those to a new filesForThirdPass option. This solution works for https://github.com/MiguelMadero/ember-cli-bundle-loader/pull/32 and could likely work for others as well.

For cases where dependencies are not well understood at configuration time, a better solution might be needed like what Nathan suggested on https://github.com/rickharrison/broccoli-asset-rev/issues/29

[MiguelMadero]

ping @rickharrison