Add support for latex fragments

org/testdata/latex.html

@@ -0,0 +1,45 @@
+<p>
+without latex delimiters the <code class="verbatim">_{i=1}</code> and <code class="verbatim">_n</code> in <code class="verbatim">\sum_{i=1}^n a_n</code> are interpreted as subscripts.
+we support <code class="verbatim">\(...\)</code>, <code class="verbatim">\[...\]</code>, <code class="verbatim">$$...$$</code> and <code class="verbatim">\begin{$env}...\end{$env}</code> as latex fragment delimiters.
+</p>
+<ul>
+<li>
+<p>
+∑<sub>i=1</sub>^n a_n (without latex delimiter)
+</p>
+</li>
+<li>
+<p>
+\(\sum_{i=1}^n a_n\)
+</p>
+</li>
+<li>
+<p>
+\[\sum_{i=1}^n a_n\]
+</p>
+</li>
+<li>
+<p>
+$$\sum_{i=1}^n a_n$$
+</p>
+</li>
+<li>
+<p>
+\begin{xyz}\sum_{i=1}^n a_n\end{xyz}
+</p>
+</li>
+<li>
+<p>
+\begin{xyz}
+\sum_{i=1}^n a_n
+\end{xyz}
+</p>
+</li>
+<li>
+<p>
+\begin{xyz}
+foobar
+\end{xyz}
+</p>
+</li>
+</ul>

org/testdata/latex.org

@@ -0,0 +1,15 @@
+without latex delimiters the =_{i=1}= and =_n= in =\sum_{i=1}^n a_n= are interpreted as subscripts.
+we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as latex fragment delimiters.
+
+- \sum_{i=1}^n a_n (without latex delimiter)
+- \(\sum_{i=1}^n a_n\)
+- \[\sum_{i=1}^n a_n\]
+- $$\sum_{i=1}^n a_n$$
+- \begin{xyz}\sum_{i=1}^n a_n\end{xyz}
+- \begin{xyz}
+  \sum_{i=1}^n a_n
+  \end{xyz}
+
+- \begin{xyz}
+  foobar
+  \end{xyz}

org/testdata/latex.pretty_org

@@ -0,0 +1,15 @@
+without latex delimiters the =_{i=1}= and =_n= in =\sum_{i=1}^n a_n= are interpreted as subscripts.
+we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as latex fragment delimiters.
+
+- \sum_{i=1}^n a_n (without latex delimiter)
+- \(\sum_{i=1}^n a_n\)
+- \[\sum_{i=1}^n a_n\]
+- $$\sum_{i=1}^n a_n$$
+- \begin{xyz}\sum_{i=1}^n a_n\end{xyz}
+- \begin{xyz}
+  \sum_{i=1}^n a_n
+  \end{xyz}
+
+- \begin{xyz}
+  foobar
+  \end{xyz}