Provide plugin to resolve globs for development

This is an extension to my previous pull request to allow resolution of file globs to a list of fully-expanded filenames by exposing a new `usemin.resolveGlobs` method. It's intended to help out with development when globs are used, since they can't be interpreted directly by a browser.

Since I didn't know whether you'd be interested in incorporating this into your plugin, this change was made with as little impact to the rest of the plugin as possible. That means there's some code duplication in there, which I would be happy to refactor if you would like to merge this feature.

If you don't want this feature in the plugin, I could publish this functionality as a separate plugin (`gulp-usemin-resolve-globs` maybe?) but it would mean I'd have to copy the section-matching and file-resolution logic out of your codebase. I figure it would be more convenient to keep them together.

Let me know if you want to see real-life usage examples.