Added Text.Pandoc.Writers.OPML.

TODO:

* Document in README
* Add tests
* Add template (and add template to cabal file)
Documented OPML writer in README.
OPML reader:  Type attributes are not case sensitive.

So, `type="link"` or `type="LINK"`.
Added OPML template, tests.

Minor fixes to OPML writer.
Improved OPML reader tests.