check-manifest shouldn't require a VCS to work

[bhearsum]

I'm currently unable to use check-manifest for a number of projects because they do not live in the root of my VCS repo. Additionally, not all projects that could benefit from check-manifest necessarily use a VCS, and among those that do, not all will necessarily use a supported one. check-manifest adds value purely from checking that MANIFEST.in covers all of the files in the source directory regardless of whether or not a VCS is used, or which one is used.

It would be great to be able to disable the VCS checks to make check-manifest work in more situations.