Add support for types

[stolen]

Typespecs are cool. It would be good to show types defined by module and types of function arguments. Need to decide how types should be shown.

• Module's types maybe will fit after module description printed by help/0

• Function arg types shouldn't be in in function header because entries like

  function(Arg1::(type1|type2|long_type_name3|undefined), Arg2::integer(), ...)
  

  are unreadable

• Maybe argument types should be listed after header like in man pages

• Maybe types should be shown only in special help(...) calls (increase arity, etc.) or even in other function like help_types(...)? Not sure.