Enhance the text backend to support configurable quote characters,

Enhance the text backend to support configurable quote characters,
configurable emphasis characters, various other configurable bits
which have been marked FIXME in the code for a while, and also to
warn when a code paragraph line is too long (because that was the
only other thing labelled FIXME). Fallback options are implemented,
and defaults set accordingly. A UTF-8 text output file now looks
like proper UTF-8.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4128 cda61777-01e9-0310-a592-d414129be87e