Allow :// in citation keys.

Closes jgm/pandoc-citeproc#166.

src/Text/Pandoc/Parsing.hs

@@ -1211,7 +1211,8 @@ citeKey = try $ do
   firstChar <- alphaNum <|> char '_' <|> char '*' -- @* for wildcard in nocite
   let regchar = satisfy (\c -> isAlphaNum c || c == '_')
   let internal p = try $ p <* lookAhead regchar
-  rest <- many $ regchar <|> internal (oneOf ":.#$%&-+?<>~/")
+  rest <- many $ regchar <|> internal (oneOf ":.#$%&-+?<>~/") <|>
+                 (oneOf ":/" <* lookAhead (char '/'))
   let key = firstChar:rest
   return (suppress_author, key)