Support hard_line_breaks markdown extension.

* Added Ext_hard_line_breaks.
* Added section in README on non-pandoc extensions.
* Exported pandocExtensions and strictExtensions in Text.Pandoc.Options.

Closes #514.

README

@@ -2171,6 +2171,16 @@ document with an appropriate header:
 
 The bibliography will be inserted after this header.
 
+Non-pandoc extensions
+=====================
+
+The following markdown syntax extensions are not enabled by default
+in pandoc, but can be enabled using the `-e` (`--enable`) option:
+
+**Extension:  `hard_line_breaks`**\
+Causes all newlines within a paragraph to be interpreted as hard line
+breaks instead of spaces.
+
 
 Producing slide shows with Pandoc
 =================================

src/Text/Pandoc/Options.hs

@@ -29,6 +29,8 @@ Data structures and functions for representing parser and writer
 options.
 -}
 module Text.Pandoc.Options ( Extension(..)
+                           , pandocExtensions
+                           , strictExtensions
                            , ReaderOptions(..)
                            , HTMLMathMethod (..)
                            , CiteMethod (..)
@@ -74,8 +76,47 @@ data Extension = Ext_footnotes
                | Ext_strikeout
                | Ext_superscript
                | Ext_subscript
+               | Ext_hard_line_breaks
                deriving (Show, Read, Enum, Eq, Ord, Bounded)
 
+pandocExtensions :: Set Extension
+pandocExtensions = Set.fromList
+  [ Ext_footnotes
+  , Ext_inline_notes
+  , Ext_pandoc_title_blocks
+  , Ext_table_captions
+  -- , Ext_image_captions
+  , Ext_simple_tables
+  , Ext_multiline_tables
+  , Ext_grid_tables
+  , Ext_pipe_tables
+  , Ext_citations
+  , Ext_raw_tex
+  , Ext_tex_math
+  , Ext_latex_macros
+  , Ext_delimited_code_blocks
+  , Ext_inline_code_attributes
+  , Ext_markdown_in_html_blocks
+  , Ext_escaped_line_breaks
+  , Ext_autolink_code_spans
+  , Ext_fancy_lists
+  , Ext_startnum
+  , Ext_definition_lists
+  , Ext_example_lists
+  -- , Ext_header_identifiers
+  , Ext_all_symbols_escapable
+  , Ext_intraword_underscores
+  , Ext_blank_before_blockquote
+  , Ext_blank_before_header
+  -- , Ext_significant_bullets
+  , Ext_strikeout
+  , Ext_superscript
+  , Ext_subscript
+  ]
+
+strictExtensions :: Set Extension
+strictExtensions = Set.empty
+
 data ReaderOptions = ReaderOptions{
          readerExtensions      :: Set Extension  -- ^ Syntax extensions
        , readerSmart           :: Bool -- ^ Smart punctuation

src/Text/Pandoc/Readers/Markdown.hs

@@ -1321,7 +1321,8 @@ endline = try $ do
   when (stateParserContext st == ListItemState) $ do
      notFollowedBy' bulletListStart
      notFollowedBy' anyOrderedListStart
-  return $ return B.space
+  (guardEnabled Ext_hard_line_breaks >> return (return B.linebreak))
+    <|> (return $ return B.space)
 
 --
 -- links

src/pandoc.hs

@@ -964,8 +964,8 @@ main = do
                      else takeDirectory (head sources)
 
   let defaultExts = if strict
-                       then Set.empty
-                       else Set.fromList [minBound..maxBound]
+                       then strictExtensions
+                       else pandocExtensions
 
   let extensions = foldl (\acc (inc,ext) -> if inc
                                             then Set.insert ext acc