Warn when adding named vertices to unnamed graphs

Or add vertex names automatically? Or both? E.g. 

``` r
make_ring(10) + vertex(1)
#> IGRAPH UN-- 11 10 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), name (v/n)
#> + edges (vertex names):
#>  [1] NA--NA NA--NA NA--NA NA--NA NA--NA NA--NA NA--NA NA--NA NA--NA NA--NA
```
