Markdown definition lists: don't require indent for first line.

Previously the body of the definition (after the `:` or `~` marker)
needed to be in column 4.  This commit relaxes that requirement,
to better match the behavior of PHP Markdown Extra.  So, now
this is a valid definition list:

    foo
    : bar

This patch also helps resolve a potentially ambiguity with table
captions:

    foo

      : bar

      -----
      table
      -----

Is "bar" a definition, or the caption for the table?  We'll count
it as a caption for the table.

Closes #2087.

src/Text/Pandoc/Readers/Markdown.hs

@@ -880,7 +880,7 @@ defListMarker = do
   tabStop <- getOption readerTabStop
   let remaining = tabStop - (length sps + 1)
   if remaining > 0
-     then count remaining (char ' ') <|> string "\t"
+     then try (count remaining (char ' ')) <|> string "\t" <|> many1 spaceChar
      else mzero
   return ()
 
@@ -916,7 +916,9 @@ defRawBlock compact = try $ do
 
 definitionList :: MarkdownParser Blocks
 definitionList = try $ do
-  lookAhead (anyLine >> optional blankline >> defListMarker)
+  lookAhead (anyLine >> optional (blankline >> notFollowedBy table) >>
+             -- don't capture table caption as def list!
+             defListMarker)
   compactDefinitionList <|> normalDefinitionList
 
 compactDefinitionList :: MarkdownParser Blocks

tests/Tests/Readers/Markdown.hs

@@ -270,6 +270,9 @@ tests = [ testGroup "inline code"
             definitionList [ (text "foo1", [para (text "bar") <>
                                             para (text "baz")])
                            ]
+          , "first line not indented" =:
+            "foo\n: bar\n" =?>
+            definitionList [ (text "foo", [plain (text "bar")]) ]
           ]
         , testGroup "+compact_definition_lists"
           [ test markdownCDL "basic compact list" $