[WIP] Replaced net/http implementation with usage of hashicorp/go-getter in order to support other url protocols (like local files),

[arizvisa]

Also updated it to allow automatically determining the checksum and type if they're not already provided.

[arizvisa]

This is could be considered not a WIP because it gets me to the next part of your plugin that's appears broken. (Or at least it's broken for me). If you agree it's broken, then this PR might be worth merging while I work more in those other areas. I can add a progress-bar really quick to this one too if you're willing to merge it despite those other areas still not working. ;)

If this plugin is not broken on latest terraform, (and it's just me being crazy, heh) then lmk and I'll fiddle with it some more... However, one thing I'd like to do sooner (rather than later) is abstract the management of the .terraform/vix directory and its sub-directories so that you aren't limited to only working with tarballs (*.box). This way support can even be added for other template types in a more generalized way.

I'd prefer to do this abstraction around the plugin's work directories first as my workflow appears very different from the way that you developed this plugin. Essentially I need this for "terraform"-ing both with and without an internet connection and am willing to do the work necessary to take it there.