non-greedy regular expression for postproc and ensure compatibility with ssio

Modify the regex

title=`grep '<h1' $1 | sed -e "s;<h1 id=\".*\">\(.*\)</h1>;\1;"`

in bin/postproc_topic.sh

to also properly match strings containing math like

<h1 id="walker-breaker-games-on-g_np">Walker-Breaker games on <span class="math inline">\(G_{n,p}\)</span></h1>

It appears that this is not trivial given that we also use SSI-directives, i.e. the server side HTML should contain something like

<!--#set var="TITLE" value="Walker-Breaker games on \(G_{n,p}\)" -->

which may need some additional escaping.