olddoc 1.5.0

olddoc now avoids changing core RDoc behavior unless it is
actually specified.  Thanks to Daniel Ferreira for noticing
this bug in [ruby-talk:437390]:

  http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/437390
  <CAPL_2g25ir4ZBJo652ao1EYB59ZvFHhpGef1V14QOV2SeVXLeA@mail.gmail.com>

We still assume any Ruby process will only use one RDoc template
in its lifetime, and not switch between them.

There are also a few minor documentation and code cleanups.

5 changes since olddoc v1.4.0:

      update URLs to HTTPS
      oldweb: apply LessHtml monkey patch late, at runtime
      oldweb: use flat_map instead of flatten.map chain
      oldweb: use native String#encode for (X)HTML escaping
      README: add NNTP archive link for mailing list