related to #5

add vc-got--program-version command to return version number

this can be used to select behaviour based on used got version.

this moves the customization options to its own group

expand the vc function status list

got diff wants a single file or two references/tags/sha.

other platforms might have different name for got or it might not be in PATH so provide means to customize it.

Add full list of vc functions which should be implemented. It is easier to track progress as all functions are in same list.

and while here adjust slightly the comment headers.

vc-got-stage is a minor mode for vc-diff buffers controlled by Got. It's a tentative at implementing the stage operation for specific changes instead of whole files. vc.el works with filesets, and this is fine. But sometimes you want to commit only certain hunks. This initial implementation allows the user to stage changes, but you can't commit them (yet).