New: --group-by=EXPR and --group-title-format=FMT

The --group-by option allows for most reports to be split up into
sections based on the varying value of EXPR.  For example, to see
register subtotals by payee, use:

  ledger reg --group-by=payee -s

This works for separated balances too:

  ledger bal --group-by=payee

Another interesting possibility is seeing a register of all the accounts
affected by a related account:

  ledger reg -r --group-by=payee

The option --group-title-format can be used to add a separator bar to
the group titles.  The option --no-titles can be used to drop titles
altogether.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/report.h b/src/report.h
index 78dc2165..6fa238f0 100644
--- a/src/report.h
+++ b/src/report.h
@@ -256,6 +256,8 @@ public:
     HANDLER(forecast_years_).report(out);
     HANDLER(format_).report(out);
     HANDLER(gain).report(out);
+    HANDLER(group_by_).report(out);
+    HANDLER(group_title_format_).report(out);
     HANDLER(head_).report(out);
     HANDLER(invert).report(out);
     HANDLER(limit_).report(out);
@@ -596,6 +598,22 @@ public:
 		  " - get_at(total_expr, 1)");
     });
 
+  OPTION__
+  (report_t, group_by_,
+   expr_t expr;
+   CTOR(report_t, group_by_) {}
+   void set_expr(const optional<string>& whence, const string& str) {
+     expr = str;
+     on(whence, str);
+   }
+   DO_(args) {
+     set_expr(args[0].to_string(), args[1].to_string());
+   });
+
+  OPTION__(report_t, group_title_format_, CTOR(report_t, group_title_format_) {
+      on(none, "%(value)\n");
+    });
+
   OPTION(report_t, head_);
 
   OPTION_(report_t, invert, DO() {