Render italic and bold in Markdown.

Scribble => Markdown
  @bold{text}    **text**
@italic{text}     _text_

collects/scribble/markdown-render.rkt

@@ -176,8 +176,7 @@
                     (if (target-url? p) p #f)))
       (define url (and targ (target-url-addr targ)))
       (begin0
-          (cond [url
+          (cond [url (define new-i
-                 (define new-i
                        (match (element-content i)
                          [(list (? string? s))
                           (element (element-style i)
@@ -191,8 +190,15 @@
                                  (eq? 'hspace (style-name s))))))
                  (parameterize ([current-preserve-spaces #t])
                    (super render-content i part ri))]
-                [else
+                [else (define style (and (element? i) (element-style i)))
-                 (super render-content i part ri)])
+                      (define bold?   (eq? style 'bold))
+                      (define italic? (eq? style 'italic))
+                      (cond [bold?   (display "**")]
+                            [italic? (display "_")])
+                      (begin0
+                          (super render-content i part ri)
+                        (cond [bold?   (display "**")]
+                              [italic? (display "_")]))])
         (when tick?
           (phrase-ticks-depth (sub1 (phrase-ticks-depth)))
           (when (zero? (phrase-ticks-depth))

collects/tests/scribble/markdown-docs/example.md

@@ -10,9 +10,9 @@
 
 [I am a hyperlink to Racket.](http://www.racket-lang.org/)
 
-Italic. \_Just underlines\_.
+_Italic_. \_Just underlines\_.
 
-Bold. \*Just asterisks.\*
+**Bold**. \*Just asterisks.\*
 
 “Dobule quoted”. ‘Single quoted’.