LaTeX reader: ignore args to column type in \multicolumn.

The `\multicolumn` command takes the column type as the second argument.
Types like `p` take an additional argument, which is now ignored and no
longer causes the table parser to fail.

Fixes: #8789

test/command/8789.md

@@ -0,0 +1,42 @@
+# `\multicolumn` with column-type `p`
+
+```
+% pandoc --from=latex --to=html
+\begin{tabular}{rrll}
+	\toprule
+	\textbf{First} & \textbf{Second} & \textbf{Third} & \textbf{Fourth} \\ \toprule
+	160 & 1 & test & test \\\midrule
+	160 & 2 & \multicolumn{2}{p{12cm}}{\textit{This is a test:}
+			\begin{equation*}
+				a^2+b^2 = c^2
+		\end{equation*}
+	}\\
+\bottomrule
+\end{tabular}
+^D
+<table>
+<thead>
+<tr class="header">
+<th style="text-align: right;"><strong>First</strong></th>
+<th style="text-align: right;"><strong>Second</strong></th>
+<th style="text-align: left;"><strong>Third</strong></th>
+<th style="text-align: left;"><strong>Fourth</strong></th>
+</tr>
+</thead>
+<tbody>
+<tr class="odd">
+<td style="text-align: right;">160</td>
+<td style="text-align: right;">1</td>
+<td style="text-align: left;">test</td>
+<td style="text-align: left;">test</td>
+</tr>
+<tr class="even">
+<td style="text-align: right;">160</td>
+<td style="text-align: right;">2</td>
+<td colspan="2" style="text-align: left;"><em>This is a test:</em> <span
+class="math display"><em>a</em><sup>2</sup> + <em>b</em><sup>2</sup> = <em>c</em><sup>2</sup></span></td>
+</tr>
+</tbody>
+</table>
+
+```