Add additional flag to output 'box' file rather than install in .vagrant.d

[epackorigan]

It would be great to add a flag to create the '.box' file for each platform, rather than 'planting' the file for use.

(tar the files into a target filename, instead of copying them to .vagrant.d)

[bacongravy]

Thanks for the suggestion, @epackorigan. As a workaround, you could try using vagrant box repackage.