Add | column separators to arrays.

This adds the ability to add `|` to a column description and have
vertical separators be added. I added types to the column descriptions
and added some logic to handle the separators when building the vertical
lists of the array.

Test plan:
 - See the Arrays screenshot looks good.
 - `make test`

test/screenshotter/ss_data.yaml

@@ -13,7 +13,7 @@
 
 Accents: \vec{A}\vec{x}\vec x^2\vec{x}_2^2\vec{A}^2\vec{xA}^2
 Arrays: |
-    \left(\begin{array}{rlc}
+    \left(\begin{array}{|rl|c||}
         1&2&3\\
         1+1&2+1&3+1\cr1\over2&\scriptstyle 1/2&\frac12\\[1ex]
         \begin{pmatrix}x\\y\end{pmatrix}&0&\begin{vmatrix}a&b\\c&d\end{vmatrix}