Add support for \intertext from amsmath and \shortintertext from mathtools

[torik42]

It would be nice to have support for \intertext from amsmath and \shortintertext from mathtools. They are used in align environment for text between two equations, e.g.

We have
\begin{align}
    a &= b \\
    \intertext{and}
    b &= c.
\end{align}

One is also allowed to skip the line break \\.

My current solution is to add

Macro(parms, '\\intertext', args='A', repl='\\text{ #1 }')

to amsmath. Only adding parms.math_text_macros.append('\\intertext') does not work because of the spacing.

I could add some tests and open a PR. Where should the tests go test_equation.py and test_display.py, or test_amsmath.py and test_mathtools.py?