Added support for a "fixed" directive

It lets you specify a fixed cost for a duration of a ledger file, for
example:

  fixed ecu $2

  2008/01/01 income
      assets:bank:checking  1 ecu
      income:salary

  end fixed

This is equivalent to:

  2008/01/01 income
      assets:bank:checking  1 ecu {=$2}
      income:salary

1 files changed, 2 insertions, 1 deletions

diff --git a/src/pool.h b/src/pool.h
index 85de73a9..995ab23c 100644
--- a/src/pool.h
+++ b/src/pool.h
@@ -123,7 +123,8 @@ public:
 
   // Parse commodity prices from a textual representation
 
-  optional<price_point_t> parse_price_directive(char * line);
+  optional<std::pair<commodity_t *, price_point_t> >
+  parse_price_directive(char * line, bool do_not_add_price = false);
 
   commodity_t *
   parse_price_expression(const std::string&          str,