pelican_comment_system: Allow restructedtext comments.

I came across this since I used restructuredtext, not Markdown, for all my articles. I figured it'd be a bit weird to have my comments in Markdown, so i went ahead and used the generate Readers class provided by Pelican instead of directly invoking the MarkdownReader. This should allow any reader that Pelican supports to be used for the pelican_comment_system plugin.