Add .frogc option `index-newest-first?`.

Default: True. When false, posts are indexed oldest first.

Truthfully, this is a hack to let me do my presentation about Frog at
RacketCon, using RacketCon. This way, each post is a "slide", and they
will appear in the order I create them. But possibly, this might be
useful for a non-traditional style of blog (I try to convince myself).
Fix bug where `xexprs->description` strings contain #\"

The practical effect is that a page could have invalid HTML and be
rendered incorrectly, due to a garbed <meta description> element.

Also, be clear about the repsective purposes of `xexpr->markdown`
vs. `xexprs->description`:

- `xexpr->markdown` may permit #\" in the string it returns -- in fact
  it _ought_ to use #\" in place of `&quot;`, `&ldquo;`, and
  `&rdquo;`. Because it's supposed to produce plain text, a subset of
  Markdown.

- By contrast `xexprs->description` is supposed to produce a string
  that can be used as an HTML attribute for a <meta> tag. Such a
  string must not contain #\" (and can't even contain backslash
  escaped #\", "\""). Instead we must use `&quot;`.