Create empty files when minifying

Htmlmin currently ignores source files that are empty or that minify to an empty result. I'm working on minifying a large project which uses a lot of HTML partials, and I'd prefer to create destination files even if they're empty in order to avoid 404 errors.

I've added a new configuration option (`copyEmptyFiles`) to control this functionality, added tests and updated documentation. Please let me know if you think this is a worthwhile feature and whether you'd like anything changed in my PR.