Literate syntax: Accept tab after '>'

Rationale:
* Haskell does not seem to even require any whitespace after '>'. Unlike
  the Haskell
  Wiki (http://www.haskell.org/haskellwiki/Literate_programming), the
  Haskell Report (http://www.haskell.org/onlinereport/literate.html)
  mentions no such restriction (though it mention other ones)
* it's very hard to "see" the reason of a syntax error due to a tab.
* the provided Emacs mode does not avoid producing tab characters in my
  setup, so indenting a line might break syntax correctness (that's how
  I noticed).