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.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/post.h b/src/post.h
index aec81e89..7111ddef 100644
--- a/src/post.h
+++ b/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);
   }