Make it possible pass in entire folders of assets

Before, if you passed in more than one specific revisioned file, this script would just remove all but the newest version. But because all files get pushed into the same Array, this gives an unwanted outcome. Also, it's kind of a drag to have to go over all your files manually (even programatically), especially if you have a lot of 'em in different places.

With this PR, I made it possible to pass in for example `['dist/**/*.*']` and still have it leave just one of each asset in the end. Please take a look at the comments in [dec2bd5](https://github.com/flovan/gulp-rev-outdated/commit/dec2bd5a7892bc6afb6f564be11469677a65beb1) for more.

Tested this with both the new approach, as well as the per-file approach and it worked in both cases. The only thing I didn't do was bump the package version, since I don't know if you want to release this as a patch or minor upgrade.