Let 'plain' option actually produce plain output.

As discussed in #1, this pull request fixes the handling of the command line option `plain` (and the subsequent use of the affected var `*plain*`). I'll have a look at escaping the contents of the `<pre>` blocks if you want to. Should be possible with something like:

```clojure
(clojure.string/escape "<title>" { \< "&lt;" \> "&gt;" \& "&amp;" })
;; => "&lt;title&gt;"
```

That would remove most problematic cases, I guess.