Added Ext_raw_html extension.

Closes #556 -- you can now specify markdown-raw_html as your
input format.  (Read: markdown minus raw_html.)

README

@@ -1854,9 +1854,13 @@ HTML, Slidy, DZSlides, S5, EPUB
 Raw HTML
 --------
 
+**Extenion: `raw_html`**
+
 Markdown allows you to insert raw HTML (or DocBook) anywhere in a document
 (except verbatim contexts, where `<`, `>`, and `&` are interpreted
-literally).
+literally).  (Techncially this is not an extension, since standard
+markdown allows it, but it has been made an extension so that it can
+be disabled if desired.)
 
 The raw HTML is passed through unchanged in HTML, S5, Slidy, Slideous,
 DZSlides, EPUB, Markdown, and Textile output, and suppressed in other

src/Text/Pandoc/Options.hs

@@ -58,6 +58,7 @@ data Extension = Ext_footnotes
                | Ext_pipe_tables
                | Ext_citations
                | Ext_raw_tex
+               | Ext_raw_html
                | Ext_tex_math_dollars
                | Ext_tex_math_single_backslash
                | Ext_tex_math_double_backslash
@@ -97,6 +98,7 @@ pandocExtensions = Set.fromList
   , Ext_pipe_tables
   , Ext_citations
   , Ext_raw_tex
+  , Ext_raw_html
   , Ext_tex_math_dollars
   , Ext_latex_macros
   , Ext_delimited_code_blocks
@@ -119,7 +121,8 @@ pandocExtensions = Set.fromList
   ]
 
 strictExtensions :: Set Extension
-strictExtensions = Set.empty
+strictExtensions = Set.fromList
+  [ Ext_raw_html ]
 
 data ReaderOptions = ReaderOptions{
          readerExtensions      :: Set Extension  -- ^ Syntax extensions

src/Text/Pandoc/Readers/Markdown.hs

@@ -729,9 +729,11 @@ htmlElement :: Parser [Char] ParserState String
 htmlElement = strictHtmlBlock <|> liftM snd (htmlTag isBlockTag)
 
 htmlBlock :: Parser [Char] ParserState (F Blocks)
-htmlBlock = return . B.rawBlock "html" <$>
-  ((guardEnabled Ext_markdown_in_html_blocks >> rawHtmlBlocks)
-     <|> htmlBlock')
+htmlBlock = do
+  guardEnabled Ext_raw_html
+  res <- (guardEnabled Ext_markdown_in_html_blocks >> rawHtmlBlocks)
+          <|> htmlBlock'
+  return $ return $ B.rawBlock "html" res
 
 htmlBlock' :: Parser [Char] ParserState String
 htmlBlock' = try $ do
@@ -1507,6 +1509,7 @@ inBrackets parser = do
 
 rawHtmlInline :: Parser [Char] ParserState (F Inlines)
 rawHtmlInline = do
+  guardEnabled Ext_raw_html
   mdInHtml <- option False $
                 guardEnabled Ext_markdown_in_html_blocks >> return True
   (_,result) <- if mdInHtml