Disable expandtab

Since `go fmt` uses tabs might as well make sure `expandtab` is off.

Edit: I don't know if this is *all* that is needed, e.g. `tabstop`, `softtabstop`, `shiftwidth`.  For instance, `ftplugin/make.vim` that comes with `vim` does `setlocal noexpandtab softtabstop=0`.