Document pretty decent Remark.js integration.

doc/plugins/contrib/remark.mdwn

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+[[!template id=plugin name=remark author="[[schmonz]]"]]
+[[!template id=gitbranch branch=schmonz/remark author="[[schmonz]]"]]
+[[!tag type/format]]
+
+[Remark.js](http://remarkjs.com) makes web slides from Markdown. This
+plugin integrates Remark into ikiwiki. Simply write `your_slides.remark`
+instead of `your_blog_post.mdwn`, separating slides with `---`.
+
+To set shared styles across all presentations on your site, put them in
+a top-level `remarklocal.css`. To set per-presentation styles, use the
+[[localstyle]] plugin as usual.
+
+## Problem: `inline`
+
+Web slides are sort of a regular web page, but not exactly: no action
+links, and almost none of the other accoutrements of `page.tmpl`. The
+branch's current behavior is that Remark slides are naively inlined like
+any other page, which -- because the Markdown is deliberately not being
+rendered by ikiwiki -- results in the slide source being displayed (and
+not elegantly). Clicking through to the slides works right, of course.
+
+Should [[inline]] (and more generally [[ikiwiki/PageSpec]]) understand
+that web slides are not exactly regular pages? And/or should this plugin
+detect when slides are being inlined and allow ikiwiki to process the
+Markdown as a sort of "preview"?
+
+## Concern: safety of web-editing
+
+Even though `remarkpage.tmpl` has no action links, is it still possible
+for someone to trick their way into web-editing a slide deck? And if
+they do, is that dangerous?