Support for Packaged Files

[benlower]

(copied from developer forum post)

I realize that unpacking/repacking a TWBX is trivial, but it's also boilerplate stuff we have to do in every solution unless it becomes core capability.

[t8y8]

My current PR #13 has a stab at this -- I can rebase it on HEAD and add tdsx support

[t8y8]

This should be closed with #44 for twbx and tdsx