Allow for setting date format with a variable.

Closes issue #44.

autoload/ledger.vim

@@ -39,7 +39,7 @@ function! ledger#transaction_date_set(lnum, type, ...) "{{{1
     return
   endif
 
-  let formatted = strftime('%Y/%m/%d', time)
+  let formatted = strftime(g:ledger_date_format, time)
   if has_key(trans, 'date') && ! empty(trans['date'])
     let date = split(trans['date'], '=')
   else

doc/ledger.txt

@@ -288,6 +288,10 @@ behaviour of the ledger filetype.
 
         let g:ledger_commodity_sep = ''
 
+* Format of transaction date:
+
+        let g:ledger_date_format = '%Y/%m/%d'
+
 * The file to be used to generate reports:
 
         let g:ledger_main = '%'

ftplugin/ledger.vim

@@ -41,6 +41,10 @@ if !exists('g:ledger_extra_options')
   let g:ledger_extra_options = ''
 endif
 
+if !exists('g:ledger_date_format')
+  let g:ledger_date_format = '%Y/%m/%d'
+endif
+
 " You can set a maximal number of columns the fold text (excluding amount)
 " will use by overriding g:ledger_maxwidth in your .vimrc.
 " When maxwidth is zero, the amount will be displayed at the far right side
@@ -288,7 +292,7 @@ function! LedgerComplete(findstart, base) "{{{1
         let update_cache = 1
       endif
     elseif b:compl_context == 'new' "{{{2 (new line)
-      return [strftime('%Y/%m/%d')]
+      return [strftime(g:ledger_date_format)]
     endif "}}}