Can not add directory as source to bag

[privatezero]

Currently it seems that only individual file inputs can be added to bags and this library fails when attempting to add a directory (which can be desirable to maintain original structure of archival data).

Unless I am mistaken, this could be resolved by tweaking FileUtils.cp to be FileUtils.cp_r at line 85 and FileUtils.rm to FileUtils.rm_r at line 95

Alternately, I suppose new add_directory and remove_directory methods could be added incorporating those changes.

Happy to make a PR for this if any of those changes seems reasonable!

[little9]

@privatezero Those changes seem reasonable. Feel free to submit a PR!