Fix latex fragment parsing

delimiters are not always 2 in length - $ being the exception. Also we have to
make sure to handle both $ and $$ delimiters.

Not proud of this solution but can't think of anything simpler right now - will
come back to this later hopefully.

org/testdata/latex.html

@@ -35,4 +35,9 @@ $$\sum_{i=1}^n a_n$$
 \end{xyz}
 </p>
 </li>
+<li>
+<p>
+$2 + 2$, $3 - 3$
+</p>
+</li>
 </ul>

org/testdata/latex.org

@@ -9,3 +9,4 @@ we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as la
 - \begin{xyz}
   \sum_{i=1}^n a_n
   \end{xyz}
+- $2 + 2$, $3 - 3$

org/testdata/latex.pretty_org

@@ -9,3 +9,4 @@ we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as la
 - \begin{xyz}
   \sum_{i=1}^n a_n
   \end{xyz}
+- $2 + 2$, $3 - 3$