Allow vhs <file> to be run without an extension

[DaltonSW]

I frequently find myself instinctively typing vhs demo if I want to run demo.tape. It seems like a reasonable assumption, especially since the extension const is already present and used when using vhs record <name>

The code change for this would be fairly negligible, and I already have a draft PR whipped up for it, but I didn't submit it since the contribution checklist indicates a discussion should be opened first :)