setDF's rownames argument could be a column to be converted to row names

Say we had

```
DT <- data.table(a = c("a", "b", "c"), b = 1:3)
```

And we wanted

```
data.frame(b = 1:3, row.names = c("a", "b", "c"))
```

It seems we could do this all by reference via:

```
rn <- DT$a
DT[ , a := NULL]
setDF(DT, rownames = rn)
```

Seems natural to implement this via

```
setDF(DT, rownames = "a")
```

Or else add an argument:

```
setDF(DT, to.row.names = "a")
```
