Add an option to start a new process instead of overriding the current one

[tbttfox]

I'm trying to use nvr with NVR_CMD=nvim-qt.exe -- on Windows, and apparently the os.execvpe replacement of the current process doesn't work with that. I found that starting nvim-qt as a separate process makes it work, so I added the option.

Unfortunately, I don't know exactly why this is an issue, but since the fix is pretty easy I didn't dig into it too far.