Add wildcard/regex functionality to migrate actions (others?)

[briandominick]

Right now, if you want to use migrate to copy several, but not all, files in a directory, you have to list them explicitly in distinct steps. This is sub-optimal for some fairly obvious reasons. I haven't run into the need for anything dramatic or dynamic (like using regex to monitor a directory for new patterns in filenames, so you can automatically include or exclude files that haven't even appeared yet), but I can imagine some use cases.

I don't know how this is best handled, if maybe we just allow the cp command for the underlying OS to handle this by its native rules, or if we declare a version of RegEx and a few limited actions such as whether to follow symlinks, delete existing files, and so forth. I can really imagine wanting all of these powers not just during a migrate action but maybe during a parse action, too.