Markdown parser: be more permissive about citation keys.

Keys may now start with an underscore as well as a letter.
Underscores do not count as internal punctuation, but are
treated like alphanumerics, so "key:_2008" will work, as
it did not before.  (This change was necessary to use keys
generated by zotero.)

Closes #1111, closes #1011.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1811,9 +1811,10 @@ citeKey = try $ do
   guard $ lastStrPos /= Just pos
   suppress_author <- option False (char '-' >> return True)
   char '@'
-  first <- letter
-  let internal p = try $ p >>~ lookAhead (letter <|> digit)
-  rest <- many $ letter <|> digit <|> internal (oneOf ":.#$%&-_+?<>~/")
+  first <- letter <|> char '_'
+  let regchar = satisfy (\c -> isAlphaNum c || c == '_')
+  let internal p = try $ p >>~ lookAhead regchar
+  rest <- many $ regchar <|> internal (oneOf ":.#$%&-+?<>~/")
   let key = first:rest
   return (suppress_author, key)