Parse footnote definitions with lazy continuation

Fixes #126

This change brings it into alignment with GitHub,
[markdown-it](https://markdown-it.github.io/#md3=%7B%22source%22%3A%22%5B%5Efoo%5D%3Abar%5Cnbaz%5Cn%5Cn%20%20%20%20quux%5Cnarst%5Cn%20%20%20%20qwfp%5Cn%5Cn%5B%5Efoo%5D%22%2C%22defaults%22%3A%7B%22html%22%3Afalse%2C%22xhtmlOut%22%3Afalse%2C%22breaks%22%3Afalse%2C%22langPrefix%22%3A%22language-%22%2C%22linkify%22%3Atrue%2C%22typographer%22%3Atrue%2C%22_highlight%22%3Atrue%2C%22_strict%22%3Afalse%2C%22_view%22%3A%22html%22%7D%7D),
and hugo/goldmark.

commonmark-extensions/src/Commonmark/Extensions/Footnote.hs

@@ -66,25 +66,31 @@ footnoteBlockSpec = BlockSpec
              updateState $ \s -> s{ counters =
                                      M.insert "footnote" (toDyn (num + 1))
                                       (counters s) }
+             isBlankLine <- option False $ try (skipWhile (hasType Spaces) >> True <$ lookAhead lineEnd)
              addNodeToStack $
                 Node (defBlockData footnoteBlockSpec){
-                            blockData = toDyn (num, lab')
+                            blockData = toDyn (num, lab', isBlankLine)
                           , blockStartPos = [pos] } []
              return BlockStartMatch
      , blockCanContain     = const True
      , blockContainsLines  = False
      , blockParagraph      = False
-     , blockContinue       = \n -> try $ do
-             () <$ (gobbleSpaces 4)
-               <|> (skipWhile (hasType Spaces) >> () <$ lookAhead lineEnd)
-             pos <- getPosition
-             return $! (pos, n)
+     , blockContinue       = \(Node root children) -> try $ do
+         let (num, lab', needsIndented) = fromDyn (blockData root) (1 :: Int, mempty :: Text, False)
+         isBlankLine <- option False $ try (skipWhile (hasType Spaces) >> True <$ lookAhead lineEnd)
+         if needsIndented && not isBlankLine then
+            gobbleSpaces 4
+         else
+            gobbleUpToSpaces 4
+         pos <- getPosition
+         let footnoteData = toDyn (num, lab', isBlankLine)
+         return $! (pos, Node root{ blockData = footnoteData} children)
      , blockConstructor    = \node ->
           mconcat <$> mapM (\n ->
               blockConstructor (blockSpec (rootLabel n)) n)
            (subForest (reverseSubforests node))
      , blockFinalize       = \(Node root children) parent -> do
-         let (num, lab') = fromDyn (blockData root) (1, mempty)
+         let (num, lab', _indented) = fromDyn (blockData root) (1, mempty, False)
          st <- getState
          let mkNoteContents refmap =
                runParserT

commonmark-extensions/test/footnotes.md

@@ -196,3 +196,59 @@ second</a>
 [^third
 fourth]</p>
 ````````````````````````````````
+
+Only the first line of a footnote's following paragraph needs indented.
+
+```````````````````````````````` example
+[^foo]:bar
+baz
+
+    quux
+arst
+    qwfp
+
+[^foo]
+.
+<p><sup class="footnote-ref"><a href="#fn-foo" id="fnref-foo">1</a></sup></p>
+<section class="footnotes">
+<div class="footnote" id="fn-foo">
+<div class="footnote-number">
+<a href="#fnref-foo">1</a>
+</div>
+<div class="footnote-contents">
+<p>bar
+baz</p>
+<p>quux
+arst
+qwfp</p>
+</div>
+</div>
+</section>
+````````````````````````````````
+
+Lazy continuations require the first line to have text in it,
+and to lazily continue a paragraph after the first, it will need to
+start with an indented line also.
+
+```````````````````````````````` example
+[^foo]:
+baz
+
+    quux
+
+[^foo]
+.
+<p>baz</p>
+<pre><code>quux
+</code></pre>
+<p><sup class="footnote-ref"><a href="#fn-foo" id="fnref-foo">1</a></sup></p>
+<section class="footnotes">
+<div class="footnote" id="fn-foo">
+<div class="footnote-number">
+<a href="#fnref-foo">1</a>
+</div>
+<div class="footnote-contents">
+</div>
+</div>
+</section>
+````````````````````````````````