display of data.table (and matrix) objects has poor alignment

Library `data.table`'s default print method intentionally truncates table output for speed, basically concatenating `head` and `tail`.

In Zeppelin, this leads to poor alignment:

```

%r
library(data.table)
DT = data.table(a = seq_len(1e6))

DT

```

I suspect this is the same issue as plagues matrix printing since `data.table` converts the object to be printed to a `matrix` first:

```
%r
m = matrix(NA, 1e4, 10)
m
```