Add support for "zip" externals.

[ferronn-dev]

These are identified with type=zip or with a .zip extension on the url. The files present are extracted and treated as a checkout. If the zip file is extracted into a directory with the same name as the basename of the external, that extra directory is removed.

[ferronn-dev]

Friendly ping. Let me know if there's some process to follow for this. Thanks!

[nebularg]

I'll try to set aside some time in the next few days for some packager love, looks good at a glance

[CDAGaming]

9 months later : D