Added lists_without_preceding_blankline extension.

* Added `Ext_lists_without_preceding_blankline` to
  `Extension` in `Options`.  Added this option to
  `githubMarkdownExtensions`.
* Made markdown reader sensitive to this.
* Closes #972.

README

@@ -2454,6 +2454,10 @@ in pandoc, but may be enabled by adding `+EXTENSION` to the format
 name, where `EXTENSION` is the name of the extension.  Thus, for
 example, `markdown+hard_line_breaks` is markdown with hard line breaks.
 
+**Extension:  `lists_without_preceding_blankline`**\
+Allow a list to occur right after a paragraph, with no intervening
+blank space.
+
 **Extension:  `hard_line_breaks`**\
 Causes all newlines within a paragraph to be interpreted as hard line
 breaks instead of spaces.

src/Text/Pandoc/Options.hs

@@ -80,6 +80,7 @@ data Extension =
     | Ext_link_attributes     -- ^ MMD style reference link attributes
     | Ext_autolink_bare_uris  -- ^ Make all absolute URIs into links
     | Ext_fancy_lists         -- ^ Enable fancy list numbers and delimiters
+    | Ext_lists_without_preceding_blankline -- ^ Allow lists without preceding blank
     | Ext_startnum            -- ^ Make start number of ordered list significant
     | Ext_definition_lists    -- ^ Definition lists as in pandoc, mmd, php
     | Ext_example_lists       -- ^ Markdown-style numbered examples
@@ -169,6 +170,7 @@ githubMarkdownExtensions = Set.fromList
   , Ext_intraword_underscores
   , Ext_strikeout
   , Ext_hard_line_breaks
+  , Ext_lists_without_preceding_blankline
   ]
 
 multimarkdownExtensions :: Set Extension

src/Text/Pandoc/Readers/Markdown.hs

@@ -1559,6 +1559,7 @@ endline :: MarkdownParser (F Inlines)
 endline = try $ do
   newline
   notFollowedBy blankline
+  guardDisabled Ext_lists_without_preceding_blankline <|> notFollowedBy listStart
   guardEnabled Ext_blank_before_blockquote <|> notFollowedBy emailBlockQuoteStart
   guardEnabled Ext_blank_before_header <|> notFollowedBy (char '#') -- atx header
   -- parse potential list-starts differently if in a list: