Only escape ampersands if they aren't the start of an escape seq.

e.g. &rsquo; now correctly makes it through jade and displays in
browser as a right single quote. An ampsersand on its own will
still be properly escaped.

Uses negative look-ahead in the regex.

Before:
&amp; -> &amp;amp;
& -> &amp;
&rsquo; -> &amp;rsquo;

Now:
&amp; -> &amp;
& -> &amp;
&rsquo; -> &rsquo;