Support vertical lines in arrays

```latex
\begin{array}{rrr|r}
1 & 0 & 0 & x\\
0&1&0&b\\
0&0&1&c
\end{array}
```
Currently we get
```
\begin{array}{rrr|r}
^
unexpected "|"
expecting letter, white space or "}"
```
It would be nice if we could represent these vertical lines. (This would require changing the EArray type.) Second best, perhaps the `|` could just be skipped?