Org reader: allow ":" in property drawer keys

Any non-space character is allowed as property drawer key, including ":"
itself (so it is not really a delimiter). The real delimiter is a space
character, so in a drawer like

:PROPERTIES:
::k:ey:: value
:END:

":k:ey:" is a key with value "value".

This usage can be seen in the Org Manual at
https://orgmode.org/manual/Using-Header-Arguments.html,
where the Org snippet

* Heading
  :PROPERTIES:
  :header-args:clojure:    :session *clojure-1*
  :header-args:R:          :session *R*
  :END:

is listed as an example.

src/Text/Pandoc/Readers/Org/DocumentTree.hs

@@ -405,7 +405,8 @@ propertiesDrawer = try $ do
 
    key :: Monad m => OrgParser m PropertyKey
    key = fmap toPropertyKey . try $
-         skipSpaces *> char ':' *> many1TillChar nonspaceChar (char ':')
+         skipSpaces *> char ':' *>
+         many1TillChar nonspaceChar (try $ char ':' *> spaceChar)
 
    value :: Monad m => OrgParser m PropertyValue
    value = fmap toPropertyValue . try $