Individual postings may each have their own payee

If a posting has the metadata field "Payee" set to a string, that will
be used as the payee name for that posting.  This affects the register
report, the payees report, and the --by-payee option.

This is useful because sometimes I send, say, 4 checks at a time to my
bank.  So on my bank statement, this is all just one amount:

   2010-06-17 Sample
     Assets:Bank        $400.00
     Income:Check1     $-100.00
     Income:Check2     $-100.00
     Income:Check3     $-100.00
     Income:Check4     $-100.00

Though it's important that the Assets:Bank posting be a single posting
of $400 value, I'd like for income reports to show whom each check came
from.  Now I can say:

   2010-06-17 Sample
     Assets:Bank        $400.00
     Income:Check1     $-100.00  ; Payee: Person One
     Income:Check2     $-100.00  ; Payee: Person Two
     Income:Check3     $-100.00  ; Payee: Person Three
     Income:Check4     $-100.00  ; Payee: Person Four

When I report this, it appears as:

10-Jun-17 Sample             Assets:Bank             $400.00     $400.00
          Person One         Income:Check1          $-100.00     $300.00
          Person Two         Income:Check2          $-100.00     $200.00
          Person Three       Income:Check3          $-100.00     $100.00
          Person Four        Income:Check4          $-100.00           0

This shows that they are all in the same transaction (which is why the
date is not repeated), but they have different payees.

src/account.cc

@@ -626,7 +626,7 @@ void account_t::xdata_t::details_t::update(post_t& post,
 
   if (gather_all) {
     accounts_referenced.insert(post.account->fullname());
-    payees_referenced.insert(post.xact->payee);
+    payees_referenced.insert(post.payee());
   }
 }
 

src/filters.cc

@@ -1042,10 +1042,10 @@ void by_payee_posts::flush()
 
 void by_payee_posts::operator()(post_t& post)
 {
-  payee_subtotals_map::iterator i = payee_subtotals.find(post.xact->payee);
+  payee_subtotals_map::iterator i = payee_subtotals.find(post.payee());
   if (i == payee_subtotals.end()) {
     payee_subtotals_pair
-      temp(post.xact->payee,
+      temp(post.payee(),
            shared_ptr<subtotal_posts>(new subtotal_posts(handler, amount_expr)));
     std::pair<payee_subtotals_map::iterator, bool> result
       = payee_subtotals.insert(temp);

src/output.cc

@@ -313,9 +313,9 @@ void report_payees::flush()
 
 void report_payees::operator()(post_t& post)
 {
-  std::map<string, std::size_t>::iterator i = payees.find(post.xact->payee);
+  std::map<string, std::size_t>::iterator i = payees.find(post.payee());
   if (i == payees.end())
-    payees.insert(payees_pair(post.xact->payee, 1));
+    payees.insert(payees_pair(post.payee(), 1));
   else
     (*i).second++;
 }

src/post.cc

@@ -123,6 +123,14 @@ optional<date_t> post_t::effective_date() const
   return date;
 }
 
+string post_t::payee() const
+{
+  if (optional<value_t> post_payee = get_tag(_("Payee")))
+    return post_payee->as_string();
+  else
+    return xact->payee;
+}
+
 namespace {
   value_t get_this(post_t& post) {
     return scope_value(&post);
@@ -160,7 +168,7 @@ namespace {
   }
 
   value_t get_payee(post_t& post) {
-    return string_value(post.xact->payee);
+    return string_value(post.payee());
   }
 
   value_t get_note(post_t& post) {

src/post.h

@@ -112,6 +112,8 @@ public:
   virtual date_t actual_date() const;
   virtual optional<date_t> effective_date() const;
 
+  string payee() const;
+
   bool must_balance() const {
     return ! has_flags(POST_VIRTUAL) || has_flags(POST_MUST_BALANCE);
   }

src/report.h

@@ -816,8 +816,9 @@ public:
          " %(justify(scrub(display_total), total_width, "
          "    4 + meta_width + date_width + payee_width + account_width"
          "      + amount_width + total_width + prepend_width, true, color))\n%/"
-         "%(justify(\" \", 2 + date_width + payee_width))"
-         "%$3 %$4 %$5\n");
+         "%(justify(\" \", date_width))"
+         " %(justify((has_tag(\"Payee\") ? payee : \" \"), payee_width))"
+         " %$3 %$4 %$5\n");
     });
 
   OPTION(report_t, related); // -r