Add an optional argument (key= ...) to as.data.table

This is for convenience, a bit tired of writing:

``` R
dt = as.data.table(x)
setkeyv(dt, c("s1", "s2"))
dt
```

I would be much nicer to have

``` R
dt = as.data.table(x, key = c("s1", "s2"))
```

Thank you,
You are doing amazing work with `data.table`.
