Added --date, similar to --account and --payee

This lets you do things like store a date as the value of a tag, then
run:

  ledger --date='has_tag("Foo") ? to_date(tag("Foo")) : date' reg

1 files changed, 6 insertions, 0 deletions

diff --git a/src/item.h b/src/item.h
index 84385eb7..13332f0b 100644
--- a/src/item.h
+++ b/src/item.h
@@ -172,6 +172,12 @@ public:
   virtual optional<date_t> effective_date() const {
     return _date_eff;
   }
+  virtual void set_date(const date_t& date) {
+    if (use_effective_date)
+      _date_eff = date;
+    else
+      _date = date;
+  }
 
   void set_state(state_t new_state) {
     _state = new_state;