Quickbook: Escapes in syntax highlighted comments.

Not great since it closes the comments around the escaped code. But to
nest it properly will require a bigger change. Would also be nice to
deal with escape in other contexts - such as strings.

[SVN r71098]

test/code-block-python.quickbook

@@ -0,0 +1,8 @@
+[article Python Code Blocks
+[quickbook 1.5]
+[source-mode python]
+]
+
+    # No escape
+    # Escape: ``*bold*``
+    # Escape: ``_underline_````/italic/``