Fix the behaviour of constructions like \e{index \i{term}} -- the index tag

Fix the behaviour of constructions like \e{index \i{term}} -- the index tag
was causing emphasis to be broken (particularly noticeable in text-like
backends). There are some instances of this in the PuTTY manual, for instance.

Unfortunately, \k references in similar situations still aren't quite right,
but fixing that will be more involved, and I haven't found any instances
yet.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@7049 cda61777-01e9-0310-a592-d414129be87e