Add headless option

[fkorotkov]

Fixes #9

[fkorotkov]

I think it's OK to default it to run with graphics for people just trying out tart. Plus setting default in mapstructure is not that trivial as expected.

Or maybe we can call the option Graphics to closer represent how tart calls it internally and not look at how VMWare does it as @trodemaster mentioned in #9. @edigaryev @trodemaster what do you both think?

[trodemaster]

Calling the option headless matches a convention seen in most of the existing packer plugins that use desktop computer hypervisors. As a user having the default show the tart GUI is desirable. Optionally hiding it is nice when doing builds as part of an automation workflow.