``mathtext`` now supports ``\substack``

``\substack`` can be used to create multi-line subscripts or superscripts within an equation.

To use it to enclose the math in a substack command as shown:

.. code-block::

.. mathmpl::

\sum_{\substack{1\leq i\leq 3\\ 1\leq j\leq 5}}

``mathtext`` now supports ``\middle`` delimiter

The ``\middle`` delimiter has been added, and can now be used with the

``\left`` and ``\right`` delimiters:

To use the middle command enclose it in between the ``\left`` and

``\right`` delimiter command as shown:

.. code-block::

.. mathmpl::

\left( \frac{a}{b} \middle| q \right)