Add xexprs->scribble-pres. Closes #2.

Given a list of xexprs representing valid HTML, return a Scribble
representation: A list of pre-part? pre-flow? or pre-content?
acceptable to Scribble's `decode`.

Although this could be generalized, currently it's only intended to
handle the subset of HTML that read-markdown returns.

Example usage:

(require scribble/base-render
         (prefix-in html: scribble/html-render)
         racket/runtime-path
         "main.rkt")

(define work-dir (find-system-path 'temp-dir))

(define (build-html-doc docs dest-file)
  (let* ([renderer (new (html:render-mixin render%) [dest-dir work-dir])]
         [fns      (list (build-path work-dir dest-file))]
         [fp       (send renderer traverse docs fns)]
         [info     (send renderer collect  docs fns fp)]
         [r-info   (send renderer resolve  docs fns info)])
    (send renderer render docs fns r-info)
    (send renderer get-undefined r-info)))

(define-runtime-path test.md "test/test.md")
(define part (~> (with-input-from-file test.md read-markdown)
                 xexprs->scribble-pres
                 decode))
(build-html-doc (list part) "test.html")
(require net/sendurl)
(send-url (str "file://" work-dir "test.html"))